Localization of a Radioactive Source

ABSTRACT

An angle-responsive sensor, comprising:
         a radiation detector adapted to detect ionizing radiation;   at least one radiation absorbing element arranged to block radiation from reaching said detector in a manner dependent on a relative orientation of a radiation source, said detector and said element, said detector and said element defining an aim for said sensor; and   circuitry coupled to said detector and which generates an output signal which varies as a function of said relative orientation,   wherein said detector and said element are arranged to have a working volume of at least 10 cm in depth and having an angular width, such that said signal defines an accuracy of better than 3 mm within one standard deviation, over said working volume.

RELATED APPLICATIONS

This Application claims benefit under §119(e), directly or indirectly,from U.S. Provisional Applications:

-   -   60/773,931 filed on Feb. 16, 2006, entitled “Radiation Oncology        Application”;    -   60/804,178 filed on Jun. 8, 2006, entitled “Radioactive Medical        Implants”;    -   60/773,930 filed Feb. 16, 2006, entitled “Localization of a        Radioactive Source”;    -   The disclosures of these applications are fully incorporated        herein by reference.

This application is a continuation-in-part of:

-   -   PCT/IL2005/000871 filed on Aug. 11, 2005, entitled “Localization        of a Radioactive Source within a Body of a Subject”; the        disclosures of which are fully incorporated herein by reference.

This Application is related to:

-   -   U.S. Provisional Application 60/600,725 filed on Aug. 12, 2004,        entitled “Medical Navigation System Based on Differential        Sensor”;    -   U.S. Provisional Application 60/619,792 filed on Oct. 19, 2004,        entitled “Using a Catheter Or Guidewire Tracking System to        Provide Positional Feedback for an Automated Catheter or        Guidewire Navigation System”;    -   U.S. Provisional Application 60/619,897 filed on Oct. 19, 2004,        entitled “Using a Radioactive Source as the Tracked Element of a        Tracking System”;    -   U.S. Provisional Application 60/619,898 19 Oct., 2004, entitled        “Tracking a Catheter Tip by Measuring its Distance From a        Tracked Guide Wire Tip”;    -   International Patent Application PCT/IL2005/001101 filed on Oct.        19, 2005; entitled “Tracking a Catheter Tip by Measuring its        Distance From a Tracked Guide Wire Tip”;    -   An International Patent Application, filed on even date with        this application, Docket No. 503/05135, entitled “Medical        Treatment System and Method”;    -   US patent application, filed on even date with this application,        Docket No. 503/05283, entitled “Medical Treatment System and        Method”;    -   US patent application, filed on even date with this application,        Docket No. 503/05413, entitled “Medical Treatment System and        Method”;

The disclosures of these applications are fully incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to location and tracking of a source ofionizing radiation, for example within a body of a subject.

BACKGROUND OF THE INVENTION

Existing techniques for intrabody tracking include direct video imagingusing a laparoscope; fluoroscopy (performance of the procedure undercontinuous or periodic X-Ray imaging); electromagnetic tracking, opticaltracking, computerized tomography (CT) tracking and ultrasonic imageassisted tracking. Some of these techniques explicitly avoid ionizingradiation. Those techniques which employ ionizing radiation, such asfluoroscopy and CT, require sufficient amounts of ionizing radiationthat radiation exposure for subjects and medical staff is a subject ofconcern.

Some applications which require intrabody tracking, such as cardiaccatheterization, typically use concurrently acquired images because thetissue through which the tracked medical device is being navigated movesfrequently. Other applications which require intrabody tracking, such asintracranial procedures, are more amenable to the use of pre-acquiredimages because the relevant tissue is relatively static.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the present invention relates to usingionizing radiation from a source in order to detect its position,optionally in or near the body of a subject, without production of animage. Optionally, the source is integrally formed with or attached to amedical device. Medical devices include, but are not limited to, tools,implants, navigational instruments and ducts.

In an exemplary embodiment of the invention, position of the source isdetermined by non-imaging data acquisition. For purposes of thisspecification and the accompanying claims, the phrase “non-imaging”indicates data acquired independent of an image acquisition process thatincludes the source and anatomical or other non-source features in asame image.

Optionally, position is determined using a sensor which has angularsensitivity resulting in a detectable change in output resulting fromradiation detection according to an effective angle of incidence ofradiation from the source. Greater sensitivity in effective angle ofincidence provides greater efficiency of the position determination interms of speed and accuracy. Embodiments with an angular range of lessthan ±100 milliradians, optionally less than ±50 milliradians aredisclosed. In an exemplary embodiment of the invention, greatersensitivity to effective angle of incidence can be achieved by moving aradiation detector and/or a shield.

Optionally, the source of ionizing radiation has an activity in therange of 0.01 mCi to 0.5 mCi. Optionally, the source of ionizingradiation has an activity less than 0.1 mCi. Optionally, the source ofionizing radiation has an activity of about 0.05 mCi. In an exemplaryembodiment of the invention, a radiation source which poses nosignificant health risk to a patient (i.e. short term exposure) and/ormedical personnel (i.e. long term exposure) may be employed.

Optionally, the refresh rate for the location data insures that thelocational information is temporally well correlated to the actuallocation of a tracked object (e.g. medical device). Recommended refreshrates vary according to the speed at which the tracked object moves andaccording to the environment in which the tracked object moves. In anexemplary embodiment of the invention, for tracking of medical devicesthrough body parts which are more static, such as brain or digestivetract, lower refresh rates, for example 10 times/second may be adequate.In some embodiments for tracking of medical devices through body partswhich move frequently, such as the heart, higher refresh rates, forexample 20 times/second may be desirable. Optionally, gating to an ECGoutput may be implemented so that positions from selected cardiac cyclephases are plotted. Other exemplary refresh rates are higher than 30 Hz,and values intermediate 0.1 Hz, 1 Hz, 3 Hz, 10 Hz and 20 Hz.

Optionally, the RMS error of a calculated position of the source ofionizing radiation is less than 10 mm, optionally less than 5 mm,optionally less than 2 mm, optionally less than 1 mm, optionally 0.5 to0.8 mm or better.

Variables which may influence the accuracy of determined position(s)include activity of the source in DPM, the accuracy and/or response timeof radiation sensors employed for detection, and the speed of theimplanted medical device. Improvement in one or more of these variablesmay compensate for one or more other variables. Optionally, reducing thespeed of a tracked medical device may be employed to compensate forother variables. Optionally, location information is displayed in thecontext of anatomical imaging data. Optionally, relevant anatomicalfeatures are highlighted to facilitate navigation of the medical deviceby medical personnel. Optionally, determined positions may be displayedin the context of a separately acquired image.

Optionally, two or more sources may be tracked concurrently. Optionally,multi-source tracking is used in determining orientation of anasymmetric medical device. Optionally, multi-source tracking is used incoordinating activity of two or more medical devices for a medicalprocedure.

An aspect of some embodiments of the present invention relates to usinga sensor with angular sensitivity (e.g., an angle-response sensor) todetect a direction towards a source of ionizing radiation. Optionally,two or three or more directions are determined, either concurrently orsuccessively, so that a position may be determined by calculating anintersection of the directions. If three or more directions areemployed, the location may be expressed as a three dimensional position.Optionally, a direction is used to determine a plane in which the sourceresides.

Optionally, sensors for detection of radiation from the source achievethe desired angular sensitivity by rotation of at least a portion of thesensor about an axis through a rotation angle. For example, detectors orradiation shields may be rotated. Alternately or additionally, sensorsmay achieve the desired angular sensitivity by translational motion.

An aspect of some embodiments of the present invention relates to asensor with an angular sensitivity which causes changes in an outputsignal from at least one radiation detector in response to an effectiveangle of incidence between the detector and a source. A target value ofthe output signal is achieved at an angle indicating the directiontowards the source. The direction is optionally used to determine aplane in which the source resides.

Optionally the sensor may include more than one radiation detector, eachradiation detector having a separate output signal. Optionally, one ormore radiation shields may be employed to shield or shadow at least aportion of at least one of the radiation detectors from incidentradiation. The degree of shielding changes as deviation from the angleindicating a direction towards the source occurs and the output signalvaries according to the degree of shielding.

Optionally, multiple radiation shields are employed in concert to form acollimator. The radiation shields may be either parallel to one anotheror skewed inwards. Optionally, the multiple radiation shield, whetherparallel or skewed, may be rotated.

Optionally, the deviation from target output is 1% of the output rangeper milliradian of angular displacement away from an angle indicating adirection towards the source. Optionally deviation in output indicatesdirection of deviation as well as magnitude of deviation. According tovarious embodiments of the invention, radiation detectors and/orradiation shields may be displaced to impart angular sensitivity. Thisdisplacement may be rotational and/or translational.

In an exemplary embodiment of the invention, the sensor is designed toprovide a useful angle dependent signal over a range of angles for arange of distances (e.g., a working volume). In an exemplary embodimentof the invention, the useful signal gives an accuracy of positioning ofa source of better than 3 mm, better than 2 mm, better than 1 mm, orbetter, for example, 0.8 mm or better, when measured within one standarddeviation. In an exemplary embodiment of the invention, the sensorincludes a collimator. In an exemplary embodiment of the invention, thisaccuracy is an average accuracy over a tracking volume. Alternatively oradditionally, the accuracy is a typical accuracy. Alternatively oradditionally, the accuracy is a worst accuracy over the volume.

In an exemplary embodiment of the invention, the sensor and/or thecollimator are designed so that different parts of the sensor have adifferent incidence angle to the source at which the signal is maximal(and/or minimal). Optionally, a composite signal from the sensorincludes the contributions of multiple such parts.

In an exemplary embodiment of the invention, the sensor is designed fora particular working volume. The sensor includes at least two parts, onepart having a maximum signal when aimed at a first target positionrelative to a center of said working volume and another part having amaximum signal when aimed at a second target position relative to acenter of said working volume. Thus, the two parts cannot have a maximalsignal at a same time. The angle of the sensor to the source isdetermined by a function of the signals from the two parts. Optionally,the sensor is designed with more than two maximal signal aiming targets,for example, three, four or more. In an exemplary embodiment of theinvention, four such areas are used to provide both X and Y angularposition indication using a single sensor.

In an exemplary embodiment of the invention, the sensor designtrades-off an accuracy of angle/position determination achievable whenthe sensor is aimed at the target (e.g., at a location of maximumaccuracy), with an accuracy for the working volume as a whole.Optionally, the two accuracies are about the same. Alternatively a ratioof between 1:4 and 4:1 is provided, for example, 1:2, 2:1 orintermediate values. Optionally, smaller or larger ratios are provided,for example depending on the application. In an exemplary embodiment ofthe invention, such ratios between accuracies are provided over a rangeof angles suitable for viewing a small translation, such as 5 mm, 10 mm,20 mm or smaller, intermediate or larger values. Optionally, thetranslation is viewed at a distance of between 10 and 100 cm, forexample, between 20 and 40 cm, or smaller or intermediate or largerdistances.

In an exemplary embodiment of the invention, the sensor is used to tracktargets with a spatial layout of sources, optionally at differentenergies. Optionally, a sensor with a single or multiple aiming pointsgenerates different signals for different sources and the relativeposition of the sources is determined by the relative positions of theenergy deposition on a suitable detector. Optionally, with a sensorhaving two aiming points, the peaks of the different sensor parts may befor different energies, reflecting the difference in relative spatiallayout between the sources and the sensors. In one example, a catheterwith two nearby sources at different energies is detected with therelative positions indicating the catheter (or other tool) orientation.It should be noted that in some embodiments of the invention, radiationfrom two nearby sources, can be detected simultaneously as both generatesubstantial counts on the sensor. This may be practiced with othersensors having a useful working volume.

An aspect of some embodiments of the present invention relates to acomputerized system for locating a medical device, optionally within abody of a subject by using angular sensitivity of a sensor module todetermine a direction. The sensor module measures incident radiation onone or more radiation detectors. Incident radiation produces an outputsignal which is translated to directional information by the system.Optionally, the directional information includes both direction andangle of offset from the direction. In an exemplary embodiment of theinvention, the directional information is used to close a control loop.Optionally, the loop is designed so that the device or other sourceremains within a range of angles determinable by the sensor. Optionally,if the amount of angular motion of the device is smaller than theangular range of the sensors, the sensor is not moved and/or notrotated.

In an exemplary embodiment of the invention, the control loop includesrotation and/or translation of the sensor, so as to maintain the deviceand/or other source within a more accurate viewing/working volume of thesensor. Optionally, the maintaining takes into account a maximumexpected motion of the device/source within a tracking period and/or anaccuracy which can be achieved at angular/translational offsets causedby such motion.

In an exemplary embodiment of the invention, the control loop and/orsensor parameters are selected to take into account an expected devicemotion rate.

An aspect of some embodiments of the invention relates to at leastpartial optimization of a radioactive sensor system to take into accounttarget motion of a radioactive source. In an exemplary embodiment of theinvention, the optimization includes trading off an angulardetermination accuracy over a range of angles, with an absolute accuracyat a certain offset angle. Optionally, this results in a sensor with auseful working volume within which a useful (e.g., for the application)accuracy is achieved.

In an exemplary embodiment of the invention, the optimization allowssensor parameters to vary in a way which changes the optimal aiming ofdifferent parts of the sensor array.

In an exemplary embodiment of the invention, one or more of thefollowing parameters of a collimator of the sensor are varied duringoptimization: slant geometry, aiming point location and/or aiming pointshape.

In an exemplary embodiment of the invention, one or more of thefollowing are provided as part of a cost function for the optimization:depth of field, motion speed, static accuracy, dynamic accuracy and/orpatient motion parameters.

In an exemplary embodiment of the invention, optimization isoptimization of a collimator design for a given sensor and usageconditions. Optionally, the optimization includes a simulation ofsignals expected to be detected and, optionally, expected noise sources.

Optionally, optimization comprises selecting a best collimator from aset of collimators, for example a set including between 3 and 20collimators, for example, between 4 and 10 collimators, for example, 5or 9 collimators. Larger, smaller and intermediate numbers ofcollimators may be provided in a set.

In an exemplary embodiment of the invention, a collimator comprises aframe including one or more slotted plates, each of which plates alignslats relative to a detector element. Optionally, the frame is adaptedfor mounting the detector thereon. Optionally, the slotted plates arereplaceable. Optionally, one or more screws and/or motors are providedfor moving, adjusting and/or calibrating the relative slat angles.Optionally, a motor is provided to rotate the frame as a whole.

An aspect of some embodiments of the invention relates to association ofa source of ionizing radiation with a medical device to facilitatedetermination of a location of the device, optionally as the device isnavigated within or near a subject's body during a medical procedure.Optionally, the source of ionizing radiation has an activity in therange of 0.01 mCi to 0.5 mCi. Optionally, the source of ionizingradiation has an activity less than 0.1 mCi. Optionally, the source ofionizing radiation has an activity of about 0.05 mCi. Associationincludes integrally forming the source and the device as a single unit.Association also includes attaching the source to the device.Optionally, the source is concentrated in an area having a largestdimension less than 10 mm, optionally less than 5 mm, optionally lessthan 2.5 mm, optionally less than 1 mm.

An aspect of some embodiments of the invention relates to use of anionizing radiation source with an activity of 0.1 mCi or less as atarget for non imaging localization or tracking, optionally in a medicalcontext. The source of ionizing radiation is selected to reduce abiological effect on the patient and/or medical personnel. Thisselection involves consideration of radiation strength, radiation typeand/or amount of exposure time (e.g. time in the body for a patientundergoing a procedure). Alternatively or additionally, radiationsources which are constructed of biocompatible material and/or coatedwith biocompatible coatings may be employed.

In an exemplary embodiment of the invention, a computerized system fortracking and locating a source of ionizing radiation is provided. Thesystem comprising:

(a) at least one non-imaging sensor module comprising at least oneradiation detector, the at least one radiation detector capable ofreceiving ionizing radiation from the radiation source and producing anoutput signal; and

(b) the CPU designed and configured to receive the output signal andtranslate the output signal to directional information.

Optionally, the source of radiation is integrally formed with orattached to a medical device.

Optionally, the at least one sensor module includes at least two sensormodules.

Optionally, the at least two sensor modules includes at least threesensor modules.

Optionally, the at least one of the at least one sensor module furthercomprises a locomotion device capable of imparting translational orrotational motion to the sensor module so that the sensor module ismoved to a new location.

Optionally, the locomotion device is operable by a translational orrotational motion signal from the CPU.

Optionally, the system additionally comprises an imaging module, theimaging module capable of providing an image signal to the CPU, the CPUcapable of translating the image signal to an image of a portion of thebody of the subject.

Optionally, the system further comprises a display device.

Optionally, the display device is capable of displaying the image of theportion of the body of the subject with a determined position of themedical device superimposed on the image of the portion of the body ofthe subject.

Optionally, the CPU receives at least two of the output signals andcomputes a position of the radiation source based on the output signals.

Optionally, the CPU receives at least three of the output signals andcomputes a position of the radiation source based on the at least threeoutput signals.

Optionally, the CPU computes the position repeatedly at intervals sothat a position of the radiation source as a function of time may beplotted.

Optionally, the radiation source employs an isotope with a half life inthe range of 6 to 18 months.

Optionally, the system further comprises the radiation source capable ofproviding the radiation.

Optionally, the directional information is produced when the source hasan activity in the range of 0.01 mCi to 0.5 mCi.

In an exemplary embodiment of the invention, a sensor for directionallylocating an ionizing radiation source is provided. The sensor comprises:

(a) at least one functional component; and

(b) a displacement mechanism which imparts angular sensitivity to thesensor by moving the at least one functional component.

Optionally, the at least one functional component comprises at least oneradiation detector, the at least one radiation detector capable ofreceiving radiation from the radiation source and producing an outputsignal;

wherein the displacement mechanism is capable of rotating the at leastone radiation detector through a rotation angle so that the outputsignal varies with the rotation angle.

Optionally, the at least one radiation detector comprises at least onefirst radiation detector and at least one second radiation detector andthe output signal comprises at least one first output signal from the atleast one first radiation detector and at least one second output signalfrom the at least one second radiation detector.

Optionally, the sensor comprises at least one radiation shield installedat a fixed angle with respect to the at least one first radiationdetector and the at least one second radiation detector so that amagnitude of the first output signal from the at least one firstradiation detector and a magnitude of the second output signal from thesecond radiation detector vary with the rotation angle.

Optionally, the sensor comprises:

(a) at least one first radiation detector and at least one secondradiation detector, each of the at least one first radiation detectorand at least one second radiation detector capable of receivingradiation from the radiation source and producing at least one firstoutput signal from the at least one first radiation detector and atleast one second output signal from the at least one second radiationdetector;

(b) at least one radiation shield, the radiation shield rotatable aboutan axis of shield rotation through an angle of shield rotation, so thata magnitude of the first output signal from the at least one firstradiation detector and a magnitude of the second output signal from thesecond radiation detector each vary with the angle of shield rotation.

Optionally, the at least one radiation shield comprises:

(i) a primary radiation shield located between the at least one firstradiation detector and the at least one second radiation detector;

(ii) at least one first additional radiation shield deployed tointerfere with incident radiation directed towards the at least onefirst radiation detector; and

(iii) at least one second additional radiation shield deployed tointerfere with incident radiation directed towards the at least onesecond radiation detector.

Optionally, wherein the at least one first additional radiation shieldand the at least one second additional radiation shield are eachinclined towards the primary radiation shield.

Optionally, wherein the at least one first radiation detector and the atleast one second radiation detector are organized in pairs, each pairhaving a first member and a second member and each radiation shield ofthe primary and additional radiation shields is located between one ofthe first member and one of the second member of one of the pairs sothat the output signal varies with the rotation angle.

Optionally, the sensor is additionally capable of revolving the at leasta functional component about an axis of revolution through an angle ofrevolution.

In an exemplary embodiment of the invention, a method of determining alocation of a device is provided. The method comprises:

(a) providing a device having a radiation source associated therewith;

(b) determining a direction towards the radiation source;

(c) further determining at least a second direction towards theradiation source;

(d) locate the device by calculating an intersection of the firstdirection and the at least a second direction.

Optionally, the further determining at least a second direction towardsthe radiation source includes determining at least a third directiontowards the radiation source and additionally comprising:

(e) calculating a point of intersection of the first direction, thesecond direction and the at least a third direction.

In an exemplary embodiment of the invention, a method of manufacturing atrackable medical device is provided. The method comprises incorporatinginto or fixedly attaching a detectable amount of a radioactive isotopeto the medical device.

Optionally, the detectable amount is in the range of 0.01 mCi to 0.5mCi.

Optionally, the detectable amount is 0.1 mCi or less.

Optionally, the detectable amount is 0.05 mCi or less.

Optionally, the isotope is Iridium-192.

An aspect of some embodiments of the invention relates to use of anionizing radiation source with an activity of 0.1 mCi or less as atarget for non imaging localization or tracking.

There is also provided in accordance with an exemplary embodiment of theinvention, an angle-responsive sensor, comprising:

a radiation detector adapted to detect ionizing radiation;

at least one radiation absorbing element arranged to block radiationfrom reaching said detector in a manner dependent on a relativeorientation of a radiation source, said detector and said element, saiddetector and said element defining an aim for said sensor; and

circuitry coupled to said detector and which generates an output signalwhich varies as a function of said relative orientation,

wherein said detector and said element are arranged to have a workingvolume of at least 10 cm in depth and having an angular width, such thatsaid signal defines an accuracy of better than 3 mm within one standarddeviation, over said working volume.

Optionally, said accuracy is better than 2 mm. Optionally, said accuracyis better than 1 mm.

In an exemplary embodiment of the invention, said signal is near linearover said working volume.

In an exemplary embodiment of the invention, a ratio between an accuracywhen said sensor is aimed at said source and when said sensor is at anangle within said working volume, is between 1:4 and 4:1.

In an exemplary embodiment of the invention, said working volume has anangular range of at least 10 milliradians.

In an exemplary embodiment of the invention, said working volume has anangular range of at least 20 milliradians.

In an exemplary embodiment of the invention, said circuitry generatessaid signal based on a combining of contributions of at least twoseparate sections of said detector. Optionally, said two sections eachhave different angular direction of maximum detection.

In an exemplary embodiment of the invention, the sensor comprises amotor configured to rotate said sensor and change its aim thereby.

In an exemplary embodiment of the invention, said circuitry generatessaid signal for a source distance of at least 10 cm.

In an exemplary embodiment of the invention, said circuitry generatessaid signal for a source distance of at least 20 cm.

In an exemplary embodiment of the invention, said working volume has arange of depths having a ratio of at least 1:2.

There is also provided in accordance with an exemplary embodiment of theinvention, a multi-focal non-imaging radiation sensor, comprising:

a detector comprising at least two distinguishable sections; and

a collimator arranged to differently collimate radiation on each of saidsections. Optionally, the sensor comprises two sections, each one with adifferent focal area. Alternatively or additionally, said collimatorprovides multiple focal points for each of said sections.

In an exemplary embodiment of the invention, said collimator allows wideangle radiation at a spatial angle of at least 10 degrees for at leasttwo sections.

In an exemplary embodiment of the invention, a focal point of a firstsection is distanced from a focal point of a second section in adirection parallel to said detector, a distance of at least 1 mm.Optionally, the sensor comprises additional sections with additionalfocal points distanced along said parallel direction. Alternatively oradditionally, said sensor has a relatively linear angular response overan angle range of at least 10 milliradians. Alternatively oradditionally, said sensor has a relatively linear angular response overa depth range of at least 10 cm.

There is also provided in accordance with an exemplary embodiment of theinvention, a method of collimator design, comprising:

defining an object range and movement rate; and

determining a collimator design responsive to said defining which has alinear-like angular response within said range and suitable for trackingsaid movement rate. Optionally, the method comprises generating acollimator according to said determining. Alternatively or additionally,the method comprises selecting a collimator according to saiddetermining.

In an exemplary embodiment of the invention, determining comprisesoptimizing.

In an exemplary embodiment of the invention, determining comprisesdetermining in response to a desired accuracy of said angular response.

There is also provided in accordance with an exemplary embodiment of theinvention, a method of collimator design, comprising:

defining an object range, movement rate and accuracy; and

determining a collimator design responsive to said defining which hassaid accuracy over said range and suitable for tracking said movementrate.

There is also provided in accordance with an exemplary embodiment of theinvention, a collimator set, comprising:

at least two collimators, each collimator having a betterangular-accuracy under a different set of conditions, each set ofconditions defining a depth and an angular range, said two setsdiffering in at least one of depth and angular range, said angularranges being greater than 10 milliradians.

In an exemplary embodiment of the invention, the set includescollimators for at least three different angular ranges.

In an exemplary embodiment of the invention, the set includescollimators for at least three different depths.

There is also provided in accordance with an exemplary embodiment of theinvention, a method of tracking a radioactive object, comprising:

(a) aiming at least one non-imaging sensor at said object;

(b) detecting an angular offset of said object from said sensor, basedon a radiation detection by said sensor;

(c) re-aiming said sensor at said object according to said angularoffset by automatic circuitry; and

(d) repeating (b)-(c) at least 10 times within a minute.

Optionally, said re-aiming does not aim said sensor exactly at saidtarget at least 50% of the time.

In an exemplary embodiment of the invention, said re-aiming comprises anestimate of a current position of the object.

In an exemplary embodiment of the invention, said re-aiming comprises anestimate of a future position of the object.

BRIEF DESCRIPTION OF FIGURES

In the Figures, identical structures, elements or parts that appear inmore than one Figure are generally labeled with the same numeral in allthe Figures in which they appear. Dimensions of components and featuresshown in the Figures are chosen for convenience and clarity ofpresentation and are not necessarily shown to scale. The Figures arelisted below.

FIG. 1 is a side view of one embodiment of a sensor module according toan exemplary embodiment of the present invention;

FIG. 2 is a schematic representation of a computerized tracking systemaccording to an exemplary embodiment of the present invention;

FIG. 3 is a side view of an additional embodiment of a sensor moduleaccording to an exemplary embodiment of the present inventionillustrating receipt of a signal by the module;

FIG. 4 is a perspective view of a computerized tracking system accordingto an exemplary embodiment of the present invention illustrating onepossible arrangement of sensor modules with respect to a patient;

FIG. 5 is a side view of another additional embodiment of a sensormodule according to an exemplary embodiment of the present invention;

FIGS. 6A and 6B are side views of further additional embodiments of asensor module according to exemplary embodiments of the presentinvention;

FIGS. 7A and 7B are graphs of simulated response time and simulated rmsposition error respectively plotted as a function of sensor rotation perphoton impact using a system according to an exemplary embodiment of thepresent invention;

FIGS. 8A and 8B are graphs of simulated response time and simulated rmsposition error respectively plotted as a function of sampling time usinga system according to an exemplary embodiment of the present invention;

FIGS. 9A and 9B are graphs of simulated response time and simulated rmsposition error respectively plotted as a function of specific activityof a radioactive signal source using a system according to an exemplaryembodiment of the present invention;

FIG. 10A is a graph of position as a function of time. Simulatedposition output from a system according to an exemplary embodiment ofthe present invention is overlaid on a plot of actual input position forthe simulation;

FIG. 10B is a graph of rms position error plotted as a function of timebased upon the two plots of FIG. 10A;

FIG. 11 is a simplified flow diagram of a method according to exemplaryembodiments of the present invention;

FIG. 12 is a graph of sensor output as a function of rotation angle;

FIG. 13 is a flowchart of a method of tracking a radioactive objectbased on angular indication, in accordance with an exemplary embodimentof the invention;

FIG. 14 is a schematic figure showing a spatial relationship between atracked object and an aiming point of a sensor, in accordance with anexemplary embodiment of the invention;

FIG. 15A is a graph showing a simulated angular response of adifferential sensor, in accordance with an exemplary embodiment of theinvention;

FIGS. 15B and 15C illustrate details of the graph of FIG. 15A;

FIG. 16 is a schematic diagram showing multiple focal aiming points fora differential sensor in accordance with an exemplary embodiment of theinvention;

FIG. 17A is a schematic diagram showing multiple non-point focal aimingpoints for an alternative differential sensor in accordance with anexemplary embodiment of the invention;

FIG. 17B is a schematic perspective illustration of a sensor showing acollimator design in accordance with an exemplary embodiment of theinvention;

FIG. 17C is a side view of the collimator of FIG. 17B, showing therelative angles and lengths of slats, in accordance with an exemplaryembodiment of the invention;

FIG. 18 is a flowchart of a method of collimator optimization and/orselection, in accordance with an exemplary embodiment of the invention;

FIGS. 19-21 illustrate exemplary detector configurations in accordancewith exemplary embodiments of the invention; and

FIGS. 22 and 23 show a real vs. a measured position and error, in anexperiment in accordance with an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

According to one embodiment of the invention (FIGS. 2 and 4), acomputerized system 40 locates and/or tracks a device. In the embodimentdepicted in FIG. 4, the device is a medical device. Medical devicesinclude, but are not limited to, tools, implants, navigationalinstruments and ducts. Tools include, but are not limited to, catheters,canulae, trocar, cutting implements, grasping implements and positioningimplements. Implants include, but are not limited to, brachytherapyseeds, stents and sustained release medication packets. Navigationalinstruments include, but are not limited to, guidewires. Ducts include,but are not limited to, tubing (e.g. esophageal tubes and trachealtubes). In exemplary embodiments of the invention, one or more movingtools are tracked.

In an exemplary embodiment of the invention, position of the source isdetermined by non-imaging data acquisition. For purposes of thisspecification and the accompanying claims, the phrase “non-imaging”indicates data not acquired as part of an image acquisition process thatincludes the source and anatomical or other non-source features in asame image. Optionally, a sensor which is not suitable for and notconnected to imaging circuitry is employed. Imaging relies uponinformation about many points, including at least one point of interest,and image analysis of the information determines characteristics of thepoint(s) of interest, for example position relative to an object. In anexemplary embodiment of the invention, position sensing providesinformation only about the source. This can improve detectability and/oraccuracy.

Optionally, the medical device is at least partially within a body of asubject 54 during at least part of the path upon which its location isdetermined. In FIG. 4, an exemplary embodiment in which system 40 isconfigured to track a device through the head of subject 54 during anintracranial medical procedure is depicted. This drawing is purelyillustrative and should not be construed as a limitation of the scope ofthe invention.

FIG. 2 shows an embodiment of system 40 including three sensor modules20 which rely on angular detection acting in concert to determine alocation of radioactive source 38. In the pictured embodiment, each ofsensors 20 determines an angle of rotation 32 indicating a directiontowards source 38. This angle of rotation 32 (FIG. 1) defines a plane inwhich source 38 resides and which crosses radiation detector 22. Angleof rotation 32 is provided as an output signal 34 which is relayed tocomputerized processing unit (CPU) 42. CPU 42 determines an intersectionof the three directions (planes) which is expressed as a point. FIG. 1also shows a rotation axis 16 around which detector 22 is rotated insome embodiments of the inventions.

According to some embodiments of the invention, a source 38 locatedwithin the boundaries 24 of detection (FIG. 1) of sensor 20 may beaccurately located by system 40 as radiation detector 22 of sensormodule 20 is rotated through a series of rotation angles 32. A source 38located outside of boundaries 24 will not be accurately located. Forthis reason, it is desirable, in some embodiments, that each of sensors20 is deployed so that the predicted path of source 38 lies withinboundaries 24. According to some embodiments of the invention, sensor 20may move to keep source 38 within boundaries 24. The size and shape ofboundaries 24 vary according to the configuration of sensor 20.

Accuracy of determination of target rotation angle 32 contributes toaccuracy of the location of source 38 as determined by system 40.Various modifications to sensor module 20 which can increase thesensitivity to small differences in rotation angle 32 are depicted asexemplary embodiments in FIGS. 3, 5, 6A and 6B and explained in greaterdetail hereinbelow.

FIG. 4 provides a perspective view of an exemplary system 40 whichemploys angular detection and includes three sensor modules 20 dispersedupon the circumference of a circle 58. In the pictured embodiment,modules 20 feature radiation shields 36. In the pictured embodiment,each module 20 rotates about an axis tangent to circle 58. This rotationallows tracking of the medical device as explained in greater detailhereinbelow. According to various embodiments of the invention,rotational motion or translational motion may be employed to facilitatethe desired angular detection. According to the embodiment depicted inFIG. 4, sensor modules 20 are situated below the head of subject 54 suchthat the vertical distance between the plane of sensor modules 20 andthe region of interest within the head is approximately equal to theradius of circle 58. This arrangement assures that each of sensors 20are deployed so that the predicted path of source 38 lies withinboundaries 24. This arrangement may be repeatably and easily achieved byproviding three of sensors 20 mounted on a board equipped with a raisedheadrest in the center of circle 58. This optionally permits a recliningchair or adjustable examination table to be easily positioned so thatsubject 54 is correctly placed relative to sensors 20 without anextensive measuring procedure.

Other arrangements of the sensors can be provided as well, for example,the sensor modules 20 can be configured in any geometric configurationin which the intersection of the planes containing their respectiverotation axes and source 38 provides observability of the 3D location ofsource 38.

In particular, FIGS. 19-21 show three exemplary alternativeconfigurations, where each rectangle represents a single sensor. FIG. 19shows a square arrangement. FIG. 20 shows an open square, absent one ofthe sensors of FIG. 19, which may be suitable if access to patient isdesired from the open side. For example, the arrangement may be 50 cm indiameter. In an exemplary embodiment of the invention, the arrangementis mounted on an arm (e.g., a gooseneck-type arm) and is positioned by auser to be, for example, near a breast (for tracking radiation therein,for example for guiding a biopsy needle). FIG. 19 is also suitable formounting on a movable/adjustable arm.

FIG. 21 shows an embodiment where the spacing between sensors in adiamond arrangement was increased, for example, to allow passage of atherapeutic beam or a tool. Additional sensors bracketing the spacingare provided.

It should be appreciated that three sensors are generally enough forposition determining. In an exemplary embodiment of the invention,additional sensors are provided so that a more accurate estimation ofposition is made, for example, by selecting a center of gravity ofpositions determined by sets of three sensors. Alternatively oradditionally, the use of more than three sensors provides an estimationof error, for example, based on the distance between the extremepossible positions reconstructed from measurements. Alternatively oradditionally, multiple sensors are provided to enhance accuracy in someparts of a positioning volume (described below as a reference 40), or toincrease such a volume.

It should also be appreciated that the sensor shape need not be arectangle, for example, a circle, square or other shapes may be used insome embodiment. In particular, a circle can be rotated around two axesto provide a line in space, rather than a plane. This may be useful forguiding a biopsy needle or a therapeutic beam or an illumination beamalong this line in space towards a radioactive source in a patient.

Positioning volume of system 40 is the set of spatial coordinates inwhich a location of source 38 may be determined. Positioning volume ofsystem 40 has a size and/or shape dependent upon positions of sensor(s)20, their design and/or their performance characteristics. Optionally,positioning volume of system 40 can be expressed as the intersection ofboundaries of detection 24 of sensors 20. Optionally, two or morepositioning volumes may be created, by using multiple sets of sensors20. Optionally, these positioning volumes may overlap.

The 3-dimensional position of the center of mass of a radiation source38 is calculated by CPU 42 from the angle 32 measured by each of sensormodules 20, given the known location and rotation axis of each ofmodules 20. According to some embodiments of the invention, source 38will be a piece of wire with a length of 1 to 10 mm. This range oflengths reflects currently available solid isotope sources 38 suppliedas wires with useful diameters and capable of providing a sufficientnumber of DPM to allow efficient operation of system 40. System 40determines the position of the middle of this piece of wire 38 andresolves the determined position to a single point, optionallyindicating margins of error.

Sensor module 20 includes at least one radiation detector 22. Radiationdetector 22 is capable of receiving radiation from radiation source 38attached to the medical device and producing an output signal 34.Radiation detector 22 may employ any technology which transformsincident radiation into a signal which can be relayed to CPU 42. Ifsource 38 is a gamma radiation source, radiation detector 22 may be, forexample, an ionization chamber, a Geiger-Mueller Counter, ascintillation detector, a semiconductor diode detector, a proportioncounter or a micro channel plate based detector. Radiation detectors 22of various types are commercially available from, for example,EVproducts (Saxonburg Pa., USA); Hammatsu Photonics (Hamamatsu City,Shizuoaka, Japan); Constellation Technology, (Largo, Fla., USA); SoltecCorporation (San Fernando Calif., USA); Thermo Electron Corporation,(Waltham Mass., USA): Bruker-biosciences (Billerica Mass., USA); SaintGobain crystals (Newbury Ohio, USA) and Silicon Sensor GMBH (Germany). Asuitable commercially available radiation detector 22 can beincorporated into the context of system 40 as part of sensor 20.Embodiments of the invention which rely upon a source 38 producing asmall number of DPM and types of detectors 22 which offer goodsensitivity (i.e. high ratio between CPM and DPM) will improve theperformance of sensor modules 20. As the distance between sensor 20 andsource 38 increases, this consideration becomes more relevant.Embodiments of the invention which rely upon source 38 with a greaterDPM output may permit use of less sensitive radiation detectors 22.

Various types of sensor modules 20 are described in greater detailhereinbelow.

System 40 further includes radiation source 38 capable of providing asufficient amount of radiation for location and/or tracking at a ratewhich will not adversely affect a procedure being carried out by themedical device. For most medical procedures, 10 locations/second issufficient to allow an operator of system 40 to comfortably navigate themedical device to a desired location. Based upon results from acomputerized simulation described in greater detail hereinbelow, theamount of radiation to meet these criteria can be made low enough thatit does not pose any significant risk to a patient undergoing aprocedure of several hours duration with source 38 inside their body.Alternatively or additionally, the amount may be made low enough so thatan operator of system 40 is not exposed to any significant risk fromradiation exposure over time as explained hereinbelow.

For example, using Iridium-192 increasing the activity of radiationsource 38 from 0.01 mCi to 0.5 mCi improves accuracy only by a factor of2 (FIG. 9B). However, activity levels below 0.1 mCi adversely affectresponse time (FIG. 9A). Activities greater than 0.1 mCi do notsignificantly improve response time. An activity of 0.05 mCi offers anacceptable trade-off between latency and accuracy as described ingreater detail hereinbelow and provides a good compromise betweenperformance and radiation dose.

A 0.05 mCi source 38 permits system 40 to achieve adequate speed andaccuracy with an amount of radiation produced so low that it may besafely handled without gloves. Radiation exposure for the patient from a0.05 mCi source 38 is only eight times greater than average absorbedbackground radiation in the United States. For purposes of comparison topreviously available alternatives, a 0.05 mCi source 38 exposes thepatient to an Effective Dose Equivalent (EDE) of 0.0022 mSv/hr. Atypical fluoroscopy guided procedure has an EDE of 1-35 mSV perprocedure and a typical Nuclear Medicine procedure has an EDE of 5 mSv.Thus, some embodiments of the invention may be employed to significantlyreduce patient radiation exposure.

Medical personnel are optionally exposed to even less radiation, withthe level of exposure decreasing in proportion to the square of theintervening distance. For example, a doctor located one meter from a0.05 mCi source 38 and performing procedures for 6 hours per day, 5 daysa week, 52 weeks a year would accumulate a total annual EDE of 0.22 mSv.This is approximately 5% of the radiation exposure level at whichexposure monitoring is generally implemented. This level of exposurecorresponds to 1.4 e⁻⁴mSV/hr which is orders of magnitude less than the1-12 mSv/hr associated with a typical dose from fluoroscopic procedures.

Iridium-192 has been used as an example because it is already approvedfor use in medical applications and is generally considered safe tointroduce into the body of a subject. However, this isotope is only anillustrative example of a suitable source 38, and should not beconstrued as a limitation of system 40. When choosing an isotope for usein the context of system 40, activity (DPM), type of radiation and/orhalf life may be considered. Activity has been discussed above. Inaddition, it is generally desired that disintegration events bedetectable with reasonable efficiency at the relevant distance, forexample 20-50 cm. Long half lives may be preferred because they makeinventory control easier and reduce total costs in the long run byreducing waste. However, short half lives may reduce concerns overradioactive materials and/or may allow smaller sources to be used.

According to some embodiments of the invention, source 38 is a source ofpositron emissions. According to these embodiments, sensors 20 determinea direction from which photons released as a result of positron/electroncollisions originate. This difference optionally does not affectaccuracy of a determined location to any significant degree because thedistance traveled by a positron away from source 38 before it meets anelectron is generally very small. Use of positrons in source 38 caneffectively amplify total ionizing radiation emissions available fordetection. Optionally, the use of multiple detector may allow thedetection of pairs of positron annihilation events to be detected. Otherexamples of source types include gamma sources, alpha sources, electronsources and neutron sources.

Regardless of the isotope, source 38 may be incorporated into a medicaldevice (e.g. guidewire or catheter) which is to be tracked.Incorporation may be, for example, at or near the guidewire tip and/orat a different location in a catheter or in an implant. The source ofionizing radiation may be integrally formed with, or attached to, aportion of the guidewire or to a portion of the medical device.Attachment may be achieved, for example by gluing, welding or insertionof the source into a dedicated receptacle on the device. Attachment mayalso be achieved by supplying the source as an adhesive tag (e.g. acrack and peel sticker), paint or glue applicable to the medical device.Optionally, the source of ionizing radiation is supplied as a solid, forexample a length of wire including a radioactive isotope. A short pieceof wire containing the desired isotope may be affixed to the guidewireor medical device. This results in co-localization of the medical deviceand the source of radiation. Affixation may be accomplished, forexample, by co-extruding the solid source with the guidewire during themanufacture of the guide wire. Alternately, or additionally, the sourceof ionizing radiation may be supplied as a radioactive paint which canbe applied to the medical device and/or the guidewire. Regardless of theexact form in which the ionizing radiation source is supplied, oraffixed to the guidewire or medical device, it should not leave anysignificant radioactive residue in the body of the subject after removalfrom the body at the end of a medical procedure.

While source 38 is illustrated as a single item for clarity, two or moresources 38 may be tracked concurrently by system 40. System 40 mayidentify multiple sources 38 by a variety of means including, but notlimited to, discrete position or path, frequency of radiation, energy ofradiation or type of radiation. According to some embodiments of theinvention, use of two or more resolvable sources 38 provides orientationinformation about the item being tracked. In other words, theseembodiments permit determination of not only a 3-dimensional positiondefined by co-ordinates X, Y and Z, but also information about theorientation of the tracked object at the defined location. This featureis relevant in a medical context when a non-symmetrical tool isemployed.

System 40 may include a channel of communication 48 capable of conveyinga data signal between the one or more sensor modules 20 and acomputerized processing unit (CPU) 42. Channel of communication may bewired or wireless or a combination thereof. Wired channels ofcommunication include, but are not limited to, direct cable connection,telephone connection via public switched telephone network (PSTN), fiberoptic connection and construction of system 40 as an integrated physicalunit with no externally apparent wires. Wireless channels ofcommunication include, but are not limited to infrared transmission,radio frequency transmission, cellular telephone transmission andsatellite mediated communication. The exact nature of channel ofcommunication 48 is not central to operation of system 40 so long assignal transmission permits the desired refresh rate. Channels ofcommunication 48 may optionally permit system 40 to be operated in thecontext of telemedicine. Alternately, or additionally, channels ofcommunication 48 may serve to increase the distance between source 38and medical personnel as a means of reducing radiation exposure to themedical personnel to a desired degree.

CPU 42 is designed and configured to receive output signal 34 viachannel of communication 48 and translate output signal 34 todirectional information concerning radiation source 38. This directionalinformation may be expressed as, for example, a plane in which radiationsource 38 resides. Output signal 34 includes at least rotation angle 32.Optionally, output signal 34 may also include a signal strengthindicating component indicating receipt of a signal from source 38.Receipt of a signal from source 38 may be indicated as either a binarysignal (yes/no) or a signal magnitude (e.g. counts per minute).According to various embodiments of the invention, output signal 34 maybe either digital or analog. Translation of an analog signal to adigital signal may be performed either by sensor module 20 or CPU 42. Insome cases, locating radiation source 38 in a single plane issufficient. However, in most embodiments of the invention, it isdesirable that CPU 42 receives two of output signals 34 and computes anintersection. If output signals 34 are expressed as planes, thisproduces a linear intersection 44 of two of the planes. This locatesradiation source 38 upon the linear intersection 44. Optionally, results44 of this calculation are displayed on a display device 43 as describedin greater detail hereinbelow. In additional embodiments of theinvention, CPU 42 receives at least three of output signals 34 andcomputes their intersection. If output signals 34 are expressed asplanes and sensors 20 are positioned on the circumference of circle 58,this produces a point of intersection 44 of at least three planes,thereby locating radiation source 38 at the calculated point ofintersection 44.

Because system 40 is most often employed to track a medical instrumentduring a medical procedure, CPU 42 is often employed to compute thepoint of intersection repeatedly at predetermined intervals so that aposition of radiation source 38 as a function of time may be plotted(see FIG. 10A). The accuracy of each plotted position and of the plot asa whole may be influenced by the activity of source 38, the accuracy andresponse time of sensors 20 and the speed at which the implanted medicaldevice is moving through subject 54. Because medical proceduresgenerally favor precision over speed, an operator of system 40 maycompensate for deficiencies in source 38, or accuracy or response timeof sensors 20, by reducing the rate of travel of the medical devicebeing employed for the procedure. FIG. 10B illustrates output of asimulated system 40 with tracking accuracy in the range of ±2 mm. CPU 42may also optionally employ channel of communication 48 to send varioussignals to sensor module(s) 20 as detailed hereinbelow. Alternately, oradditionally, CPU 42 may also optionally employ channel of communication48 to send various signals to the medical device. According to variousembodiments of the invention, system 40 may be employed in the contextof procedures including, but not limited to, angioplasty (e.g. balloonangioplasty), deployment procedures (e.g. stent placement orimplantation of radioactive seeds for brachytherapy), biopsy procedures,excision procedures and ablation procedures.

While CPU 42 is depicted as a single physical unit, a greater number ofphysically distinct CPUs might actually be employed in some embodimentsof the invention. For example, some functions, or portions of functions,ascribed to CPU 42 might be performed by processors installed in sensormodules 20. For purposes of this specification and the accompanyingclaims, a plurality of processors acting in concert to locate source 38as described herein should be viewed collectively as CPU 42.

According to some embodiments of the invention, system 40 concurrentlyemploys three or more sensor modules 20 in order to concurrently receivethree or more output signals 34 and compute three or more directionsindicating signal source 38. If the directions are expressed as planes,the three or more planes intersect in a single point. However, system 40includes alternate embodiments which employ two, or even one, sensormodule 20 to localize source 38 to a single point. This may be achievedin several different ways as described hereinbelow.

According to some embodiments of system 40 at least one of sensor module20 is capable of rotating the at least one radiation detector 22 througha series of positions. Each position is defined by a rotation angle 32so that receiving the radiation from source 38 upon detector 22 varieswith rotation angle 32. This rotation may be accomplished in a varietyof ways. For example, rotation mechanism 26 may be operated by feedbackfrom 28 from radiation detector 22 according to a rule with amount ofreceived radiation as a variable. Alternately, rotation mechanism 26 maybe operated by a signal from CPU 42 according to a rule including amountof received radiation and/or time as variables. Alternately, rotationmechanism 26 may rotate radiation detector 22 according to a fixedschedule, with no regard to how much radiation impinges upon radiationdetector 22 at any particular rotation angle 32. Rotation mechanism 26may employ a wide variety of different mechanisms for achieving rotationangle 32. These mechanisms include, but are not limited to, mechanicalmechanisms, hydraulic mechanisms, pneumatic mechanisms, electricmechanisms, electronic mechanisms and piezoelectric mechanisms.Optionally, an independent angle measuring element 30 may be employed tomore accurately ascertain the actual rotation angle 32. Although anglemeasuring element 30 is depicted as a physically distinct component inFIGS. 1, 2 and 3, it could be physically integrated into rotationmechanism 26 without affecting performance of system 40 to anysignificant degree. Regardless of the exact operational details, theobjective is to detect the rotation angle 32 at which sensor module 20is pointing directly towards source 38. This angle will be referred toas the target rotation angle 32.

According to some embodiments of system 40, radiation detector 22 (FIGS.3, 5, 6A and 6B) includes at least one first radiation detector 22A andat least one second radiation detector 22B. These embodiments of system40 rely upon comparison of output signals 34 from radiation detectors22A and 22B for each rotation angle 32. A target angle of rotation 32which produces output signals 34 from radiation detectors 22A and 22Bwith a known relationship indicates that radiation detectors 22A and 22Bare both facing source 38 to the same degree. When radiation detectors22A and 22B have identical receiving areas, the known relationship isequality. This target angle of rotation 32 is employed to determine aplane in which source 38 resides.

In order to increase the sensitivity of system 40 to small differencesbetween output signals 34 from radiation detectors 22A and 22B it ispossible to introduce one or more radiation shields 36 at a fixed anglewith respect to radiation detectors 22A and 22B. Radiation Shield 36causes a magnitude of the component of output signal 34 from firstradiation detector 22A and a magnitude of the component of output signal34 from second radiation detector 22B to each vary with rotation angle32 (see FIG. 3). Radiation shield 36 differentially shadows eitherradiation detectors 22A or 22B depending upon the relationship betweenangles of incidence 39 and 41. At some angle of rotation 32, neitherradiation detector 22A nor 22B will be shadowed by radiation shield 36.This angle of rotation 32 is employed to determine a plane in whichsource 38 resides. This configuration insures that small variations fromthis target angle of rotation 32 cause relatively large differences inthe output signals 34 from radiation detectors 22A and 22B because ofthe shadow effect. Use of radiation shield 36 in sensor module 20 canincrease the sensitivity of system 40. This increased sensitivitypermits sensor module 20 to function effectively even with a low numberof detectable radioactive counts.

FIG. 6A illustrates an additional embodiment of sensor module 20 inwhich the radiation shield includes a primary radiation shield 36located between first radiation detector 22A and second radiationdetector 22B. The picture embodiment also includes a series of firstadditional radiation shields (36A1, 36A2, and 36A3) which divide firstradiation detector 22A into a series of first radiation detectors andinterfere with incident radiation directed towards first radiationdetector 22A. The pictured embodiment also includes a series of secondadditional radiation shields (36B1, 36B2, and 36B3) which divide secondradiation detector 22B into a series of second radiation detectors andinterfere with incident radiation directed towards second radiationdetector 22B. This configuration can insure that even smaller variationsfrom target rotation angle 32 cause relatively large differences in theoutput signals 34 from radiation detectors 22A and 22B by increasing theshadow effect in proportion to the number of additional radiationshields (36A1, 36A2, 36A3, 36B1, 36B2, and 36B3 in the picturedembodiment). Use of additional radiation shields (e.g. 36A1, 36A2, 36A3,36B1, 36B2, and 36B3) in sensor module 20 may serve to achieve anadditional increase in sensitivity of system 40. Optionally, secondaryradiation shields (36A1, 36A2, 36A3, 36B1, 36B2, and 36B3 in thepictured embodiment) are inclined towards primary radiation shield 36.The angle of secondary radiation shields 36A1, 36A2, 36A3, 36B1, 36B2,and 36B3 towards primary shield 36 can be changed, for example, using amotor to improve focus and/or define imaging volume.

A similar effect may be achieved by holding radiation detectors 22A and22B at a fixed angle and subjecting radiation shield(s) 36 (FIG. 6B) toangular displacement. System 40 is optionally provided in an embodimentin which radiation detector 22 includes at least one first radiationdetector 22A and at least one second radiation detector 22B and outputsignal 34 includes discrete components from detectors 22A and 22B withat least one radiation shield 36 rotatable about an axis of shieldrotation through an angle of shield rotation 32 so that a magnitude ofdiscrete components of output signal 34 from detectors 22A and 22B eachvary as a function of the angle of shield rotation 32. Axes 16 are usedto indicate to individual rotation axes of each radiation shield.Optionally, the radiation shields are rotated as a group to track thesource, instead of rotating the sensor itself. Optionally, the radiationshields are rotated to adjust an aim of the sensor, for example, tomodify a depth range and/or a focus point and/or dispersement.Optionally, adjustment is using a screw, optionally manual, optionallyusing an actuator. Optionally, a controller is used to adjust focusingand/or aiming depending on the movement of the source relative to thesensor. Optionally, as described below, a template, such as a slottedplate, is used to position the radiation shields.

Referring now to FIG. 5, alternate embodiments of sensor module 20 ofsystem 40 are configured so that radiation detector 22 includes aplurality of radiation detectors 22 and a plurality of protrudingradiation shields 36 interspersed between the plurality of radiationdetectors 22. According to these embodiments, plurality of radiationdetectors 22 is organized in pairs, each pair having a first member 21and a second member 23 and each protruding radiation shield 36 of theplurality of protruding radiation shields is located between firstmember 21 and second member 23 of the pair of radiation detectors 22.According to this embodiment, sensor module 20 is capable of rotatingthe radiation detectors 22 through a series of rotation angles 32 sothat the receiving the radiation from radiation source 38 upon radiationdetectors 22 varies with rotation angle 32. Each radiation detectorproduces an output signal 34. CPU 42 sums output signals 34 from allfirst members 21 to produce a first sum and all second members 23 toproduce a second sum. Assuming that all of radiation detectors 22 areidentical, when the sensor is aimed directly at the center of mass ofsource 38 (target rotation angle 32), the first sum and the second sumare equivalent. This embodiment ensures that the total output for theentire module 20 increases rapidly with even a very slight change inrotation angle 32 in either direction. Alternately, or additionally, thesign of the total output for the entire module 20 indicates thedirection of rotation required to reach the desired rotation angle 32.Thus, this configuration serves to increase both speed of operation andoverall accuracy of system 40. This type of sensor module 20 may beoperated (for example) by implementation of a first algorithm summinggamma ray impacts from source 38 for a period of time and allowing CPU42 to decide, based on the sign and/or total output for the entiremodule 20, in which direction and/or to what degree to rotate radiationdetectors 22 in an effort to reach a desired rotation angle 32. Avariant of this algorithm is described below, with respect to FIG. 13.Alternately, CPU 42 may (for example) implement a second algorithm whichrotates radiation detectors 22 a very small amount in response to everydetected count. Performance data presented herein is based upon asimulation of the second algorithm, but the first algorithm is believedto be useful, at least for some applications.

In an exemplary embodiment of the invention, the interconnection of thedetectors (e.g., in the sensor of FIG. 5) is on an analog level or on adigital level. Optionally, a switching mechanism, for example, suitablemultiplexers and/or digital processing, is used to selectively addtogether or otherwise combine signals detected from individualdetectors. In one example, based on a knowledge of the approximaterelative positions of the source and sensors; and aiming direction ofindividual shields, some detector elements are given a higher weightand/or the contribution of some detectors dropped. This may be useful,for example, when different detector elements have different viewingvolumes or lines and focus of the sensor as a whole is changed byselecting detectors in accordance. Alternatively or additionally, energymay be detected for each such detector, for example, to discriminatemulti-energy sources or multiple sources with different energy (e.g., 2,3, 4, 5 sources). Optionally, for some embodiments, localization withina detector element is provided, for example, using methods know in theart. This may provide a higher accuracy than provided by the shieldsalone.

According to additional embodiments of system 40, a single sensor module20 may be employed to determine two intersecting planes in which source38 resides. This may be achieved, for example, by revolution of sensormodule 20 or by moving sensor module 20 to a new location.

According to some embodiments of the invention, sensor module 20 may beadditionally capable of revolving radiation detector 22 about an axis ofrevolution 25 through an angle of revolution 29. Revolution is producedby a revolution mechanism 27 which may function in a variety of ways asdescribed hereinabove for rotation mechanism 26. According to theseembodiments of the invention angle of revolution 29 is included as acomponent of the orientation of sensor module 20 and is included inoutput signal 34. Revolution may be employed in the context of any orall of the sensor module 20 configurations described hereinabove andhereinbelow. Revolution may occur, for example, in response to arevolution signal 46 transmitted to sensor module 20 from CPU 42 viachannel of communication 48.

According to additional embodiments of the invention, sensor module 20includes a locomotion device 31 capable of imparting translationalmotion 33 to module 20 so that the location of module 20 is changed.Locomotion may be initiated, for example, in response to a translationalmotion signal 46 transmitted to sensor module 20 from CPU 42 via channelof communication 48. According to various embodiments of the invention,locomotion may be used to either permit a single sensor module 20 tooperate from multiple locations or to provide angular sensitivity tosensor module 20. In other words, translational motion may be used as asubstitute for angular displacement, especially in embodiments whichemploy at least one radiation shield 36. In embodiments which employtranslational motion, translation of a single sensor 20 in a firstdimension permits acquisition of a first set of directional information.For example, in the embodiment of system 40 depicted in FIG. 4,successive vertical displacement of sensor 20A could be used todetermine a first plane in which source 38 resides. Successivehorizontal displacement of sensor 20B could be used to determine asecond plane in which source 38 resides. Alternately, or additionally, asingle sensor 20 may be subject to both vertical and horizontaldisplacement. Successive vertical and horizontal displacement permits asingle sensor 20 to determine two non-parallel planes in which source 38resides. Concurrent vertical and horizontal displacement along a singleline permits a single sensor 20 to determine a single plane in whichsource 38 resides. Determination of intersection of 2 or 3 or moreplanes is as determined above. Optionally locomotion and revolution maybe employed in the same embodiment of the invention.

Optionally, system 40 further includes an imaging module 50 including animage capture device 56 capable of providing an image signal 52 to CPU42. Imaging module 50 optionally includes an interface to facilitatecommunication with CPU 42. CPU 42 is capable of translating image signal52 to an image of a portion of the body of subject 54. According tovarious embodiments of the invention, imaging module 50 may rely uponfluoroscopy, MRI, CT or 2D or multi-plane or 3D angiography. Forintracranial procedures, imaging generally need not be conductedconcurrently with the procedure. This is because the brain does notshift much within the skull. Images captured a day or more before aprocedure, or a few hours before a procedure, or just prior to aprocedure, may be employed. According to alternate embodiments of theinvention, image data is acquired separately (i.e. outside of system 40)and provided to CPU 42 for alignment.

Alignment methods and the algorithms for anatomical image display andtracking information overlay are reviewed in Jolesz (1997) Radiology.204(3):601-12. The Jolesz article, together with references citedtherein, provides enablement for a skilled artisan to accomplishconcurrent display and alignment of image data and tracking data. TheJolesz reference, together with references cited therein, are fullyincorporated herein by reference to the same extent as if eachindividual reference had been individually cited and incorporated byreference.

In an exemplary embodiment of the invention, the location(s) determinedby system 40 are registered with respect to the image. This may beaccomplished, for example by registering system 40 and/or sensors 20 toimage capture device 56.

Regardless of which type of sensor module 20 is employed, system 40 mayinclude a display device 43 in communication with CPU 42. Display device43 may display the image of the portion of the body of the subject witha determined position of the medical device (corresponding to a positionof source 38) superimposed on the image of the portion of the body ofthe subject. The superimposed determined position is optionallyrepresented as a point on display screen 43. Optionally the point issurrounded by an indicator of a desired confidence interval determinedby CPU 42. The confidence interval may be displayed, for example, as acircle, as two or more intersecting lines or as one or more pairs ofbrackets. Alternately, or additionally, display device 43 may displayposition coordinates of a determined position of the medical device(e.g., corresponding to a position of source 38 at a tip of guidewire).

Display device 43 may be provided with a 3-dimensional angiographydataset from CT, MRI, or 3-D angiography, imaged either during theprocedure or prior to the procedure. Appropriate software can beemployed to extract a 3-D model of the vasculature from the angiographydataset, and display this model using standard modes of 3-D modelvisualization. A 3-dimensional graphical representation of the guidewireor catheter can be integrated into the 3-D model of the vasculature andupdated with minimal temporal delay based on the position informationprovided by system 40 to indicate the position of the guidewire orcatheter within the vasculature. The entire 3-D model including thevasculature and the catheter can be zoomed, rotated, and otherwiseinteractively manipulated by the user during performance of theprocedure in order to provide the best possible visualization.

Optionally, system 40 may further include one or more user input devices45 (e.g. keyboard, mouse, touch screen, track pad, trackball,microphone, joystick or stylus). Input device 45 may be used to adjustan image as described hereinabove on display device 43 and/or to issuecommand signals to various components of system 40 such as rotationmechanism 26, revolution mechanism 27, locomotion device 31 or imagecapture device 56.

The invention optionally includes a sensor 20 for determining a plane inwhich a radiation source resides as depicted in FIG. 3 and describedhereinabove. Briefly, the sensor 20 includes at least one radiationdetector 22, the at least one radiation detector capable of receivingradiation from radiation source 38 and producing an output signal 34.Sensor 20 is capable of rotating radiation detector 22 through a seriesof positions, each position defined by a rotation angle 32 so that thereceiving the radiation from radiation source 38 upon radiation detector22 varies with rotation angle 32. Rotation is optionally achieved asdescribed hereinabove. A rotation angle 32 which produces apredetermined and/or extreme (e.g., maximum) target output signalindicates the plane in which radiation source 38 resides.

According to some embodiments of sensor 20, radiation detector 22includes at least one first radiation detector 22A and at least onesecond radiation detector 22B and output signal 34 includes a firstoutput signal from first radiation detector 22A and a second outputsignal from radiation detector 22B.

According to some embodiments of sensor 20, at least one radiationshield 36 is further installed at a fixed angle with respect todetectors 22A and 22B. As a result, a magnitude of the first outputsignal 34 from the at least one first radiation detector and a magnitudeof the second output signal 34 from radiation detector 22B each varywith rotation angle 32 as detailed hereinabove.

A sensor 20 for determining a plane in which a radiation source residesand characterized by at least one radiation shield 36 rotatable about anaxis of shield rotation through an angle of shield rotation 32 asdescribed hereinabove in detail (FIG. 6B) is an additional embodiment ofthe invention.

Sensor 20 for determining a plane in which a radiation source resides asdepicted in FIG. 5 and described hereinabove is an additional embodimentof the invention.

According to alternate embodiments of the invention, a method 400 (FIG.11) of determining a location of a medical device within a body of asubject is provided. Method 400 includes co-localizing 401 a radioactivesignal source 38 with a medical device. Co-localization may be achieved,for example, by providing a device having a radiation source associatedtherewith or by associating a radiation source with a device.

Method 400 further includes determining 402 a first plane in which theomni directional signal generator resides, further determining 403 asecond plane in which the omni directional signal generator resides,calculating 404 a linear intersection of the first plane and the secondplane as a means of determining a line upon which the medical deviceresides.

Method 400 optionally includes further determining 405 at least oneadditional plane in source 38 resides.

Method 400 optionally includes calculating 406 a point of intersectionof the first plane, the second plane and the at least one additionalplane as a means of determining a location of the medical device.

Optionally, method 400 is successively iterated 408 so that a series oflocations are generated to track an implanted medical device in motion.Calculated locations may be displayed 410 in conjunction with anatomicalimaging data if desired.

The various aspects and features of system 40 and/or sensors 20described in detail hereinabove may be employed to enable or enhanceperformance of method 400.

System 40 and method 400 may employ various mathematical algorithms tocompute the location of source 38. One example of an algorithm suitedfor use in the context of some embodiments of the invention calculatesthe position of source 38 from sensor output signal 34, sensor position,and sensor orientation of three sensors as follows:

-   -   1) the plane defined by each sensor module 20 is calculated        using an equation of the form

Ax+By+Cz=D

-   -   2) the coefficients A, B, C, and D are calculated as follows:        -   a. Three non-collinear points are defined within system 40's            internal reference frame.        -   b. These three points are then shifted by the position of            sensor 20 and rotated by the sensor orientation. This            defines the plane in which source 38 would lie if output            signal 34 was zero.        -   c. These three points are then rotated about the axis of            rotation angle 32 of sensor 20 by rotation angle 32            indicated by output signal 34. This defines the plane in            which source 38 lies as measured by a particular sensor 20.        -   d. Using the x, y, z coordinates of the three points, x1,            y1, z1, x2, y2, z2, x3, y3, z3 in the following equations,            A, B, C, and D are calculated as follows:            -   i. A=y1(z2−z3)+y2(z3−z1)+y3(z1−z2)            -   ii. B=z1(x2−x3)+z2(x3−x1)+z3(x1−x2)            -   iii. C=x1(y2−y3)+x2(y3−y1)+x3(y1−y2)            -   iv. D=x1(y2*z3−y3*z2)+x2(y3*z1−y1*z3)+x3(y1*z2−y2*z1)    -   3) Calculation of A, B, C, and D for each of three sensors 20        produces a system of three equations in three unknowns:

${\begin{bmatrix}{A\; 1} & {B\; 1} & {C\; 1} \\{A\; 2} & {B\; 2} & {C\; 2} \\{A\; 3} & {B\; 3} & {C\; 3}\end{bmatrix}\begin{bmatrix}x \\y \\z\end{bmatrix}} = \begin{bmatrix}{D\; 1} \\{D\; 2} \\{D\; 3}\end{bmatrix}$

-   -   This system of equations can be solved to provide an exact        solution for (x, y, z) (or part of the vector), the point of        intersection of the three planes, which is the position of the        source 38.        Use of additional sensors 20 improves the accuracy by averaging        the errors in the individual sensors, and may also provide a        means of estimating the accuracy of the position measurement by        indicating the extent to which the sensors agree with each        other.        When 4 or more sensors are used, the algorithm is as follows:        Steps 1 and 2 above remain the same—the equation of the plane        indicated by each sensor is calculated. Step 3 is modified as        follows:    -   3) Once A, B, C, and D have been calculated for each of the        sensors an over-determined system of more than three equations        in three unknowns results:

${\begin{bmatrix}{A\; 1} & {B\; 1} & {C\; 1} \\{A\; 2} & {B\; 2} & {C\; 2} \\{A\; 3} & {B\; 3} & {C\; 3} \\\vdots & \vdots & \vdots\end{bmatrix}\begin{bmatrix}x \\y \\z\end{bmatrix}} = \begin{bmatrix}{D\; 1} \\{D\; 2} \\{D\; 3} \\\vdots\end{bmatrix}$

-   -   -   This over-determined system can be solved in a least square            sense using methods familiar to those skilled in the art in            order to obtain the best solution for (x, y, z), which is            the most likely position of the tracked element. There is            generally no exact solution due to the error in the sensor            outputs, there may be no single point through which all of            the planes pass.        -   In order that the least square solution may be based on the            error defined by the Euclidian distance between each plane            and the solution for (x, y, z), it is necessary to scale all            of the coefficients defining each plane by the lengths of            their respective Normal vectors (the Normal vector is the            vector defined by (A, B, C)). This is done by dividing A, B,            C, and D by sqrt (Â2+B̂2+Ĉ2) before performing the least            square solution.

    -   4) The Euclidian distance between each of the planes and the        calculated position can be used as a measure of the accuracy of        the position measurement. Once the coefficients have been scaled        by the length of the Normal vector, this distance can be        calculated for each sensor as Ax+By+Cz−D. The mean value of the        distances from each plane to the calculated position gives a        measure of the extent to which all of the sensors agree on the        position that was calculated.

Overdetermined systems of equations may be solved using least squaresolution algorithms. Suitable least square algorithms are available ascomponents of commercially available mathematics software packages.

Optionally, other methods of solving equation sets as known in the artare used. Optionally, instead of a set of equations, other calculationmethods are used, for example, neural networks, rule based methods andtable look up methods in which the signals from the sensors are used tolook-up or estimate a resulting position. In systems where the sensorsmove linearly, other solution methods may be used, for example,translating linear positions of the sensors into spatial coordinates ofthe source.

In order to increase the accuracy and performance of system 40 andmethod 400, advance calibration may optionally be performed. Theposition and orientation of each of the sensor modules 20 can becalibrated instead of relying upon values based on the mechanicalmanufacturing of the system. The calibration procedure involves usingsystem 40 to measure the 3-dimensional position of a source 38 at anumber of known positions defined to a high degree of accuracy. Sincethe position of source 38 is known, the equations normally used tocalculate the positions (described above) can now be used with thesensor positions and orientations as unknowns in order to solve forthese values. Various minimization procedures are known in the art. Thenumber of measurements needed to perform such a calibration may dependon the number of sensor modules 20 in system 40, since it is useful tomake enough measurements to provide more equations than unknowns. Thiscalibration procedure also defines the origin and frame of referencerelative to which system 40 measures the position of the source, and canoptionally provide alignment between the tracking system and anothersystem to which it is permanently attached, such as a fluoroscopy systemor other imaging system.

In an exemplary embodiment of the invention, once a position of thesource is known, the sensors can remain aimed at the source and notchange their orientation. Optionally, if the source moves, determinedfor example, by a significant change in detected radiation (e.g., a dropof 30%, 50%, 70%, 90% or a greater or intermediate drop), the sensor ismoved to scan a range of angles where the source is expected to be.Optionally, the sensor generates a signal indicating on which side ofthe sensor the source is located, for example as described below.Optionally, the range of scanning depends on an expected angularvelocity of the source, for example, based on the procedure, based onthe history and/or based on a user threshold. If scanning within therange fails, the range is optionally increased. Optionally, for exampleas described below, the sensor generates a signal indicating the angularoffset of the source.

Optionally, if multiple target sources are provided (e.g., ones withdifferent count rates and/or different energy of emission), the sensorsjump between target angles. Optionally, a steady sweep between a rangeof angles encompassing the two (or more) sources is provided.Optionally, sweeping is provided by ultrasonic or sonic vibrations ofthe sensor or part thereof, for example, comprising a range of angles 1,5, 10, 20, 50 or more times a second. Optionally, the amplitude of thevibration determines the range of angles. Optionally, the sensors orsensor portion is in resonance with one or more vibration frequencies.

Optionally, scanning of the sensors, at least in a small range ofangles, such as less than 10 or less than 5 or less than 1 degree, isprovided even when the sensor is locked on a target source.

The tracking accuracy of system 40 using Iridium-192 as source 38 asdescribed hereinabove has been evaluated only by computer simulation.The simulation is a model of the random distribution of gamma photonsemitted by a source 38 within a model head and absorbed by thephoton-sensitive elements 22 in a compound differential sensor unit 20of the type illustrated in FIG. 5. According to the simulation,radiation detector 22 of sensor module 20 rotates so that a new rotationangle 32 is defined every time a photon is absorbed by detector 22. Ifthe photon is absorbed by a positive radiation detector 21 thenradiation detector 22 of sensor module 20 rotates in the positivedirection, and if it is absorbed by a negative sensor 23 then radiationdetector 22 of sensor module 20 rotates in the negative direction. Totaloutput signal 34 of sensor module 20 is its average orientation duringthe sample time.

According to the simulation, performance is defined by two parameters,however other parameters may be used in a practical system:

-   -   1) The Root Mean Square (RMS) error when the target is        stationary    -   2) The time to indicate a 9 mm change in calculated location        after a 10 mm change in actual location of source 38.

The following parameter values are fixed in the simulation:

-   -   1) Distance from the source to the sensor=25 cm (worst case        distance)    -   2) Source distance for which sensor is geometrically        optimized=25 cm    -   3) Width of photon-sensitive surface in each sub sensor=2 mm (18        in FIG. 5)    -   4) Sensor length=10 cm (14 in FIG. 5)    -   5) Height of dividing walls between sensors=5 cm (35 in FIG. 5)    -   6) Width of dividing walls at their base=4 mm (37 in FIG. 5)    -   7) Number of subsensors defined by walls in the compound        sensor=7 (36 in FIG. 5)    -   8) Sensor sensitivity (fraction of incoming gamma rays which are        detected)=0.3        The simulation evaluated and optimized the following parameters        with respect to influence on performance:    -   1) Rotation magnitude per absorbed photon (FIGS. 7A and 7B)    -   2) Sample time (FIGS. 8A and 8B)    -   3) Photons per second (source activity level) (FIGS. 9A and 9B)

The performance parameters are measured on a simulated movement scenario(FIGS. 10A and 10B). The simulation determined that as rotation perphoton impact increases, response time is improved (FIG. 7A). However,as rotation per photon impact increases, RMS position error alsoincreases (FIG. 7B). There is clearly a trade-off between latency andaccuracy. This parameter can be modified in real-time in order tooptimize the trade-off using a motion detection algorithm as describedhereinbelow.

The simulation determined that sample time has no significant impact onlatency or accuracy (As shown, for example in FIGS. 8A and 8B). This maybe because for small values of rotation per impact, the number ofimpacts per sample has minimal effect on accuracy and only determinesthe latency (the total amount of rotation per sample). However, if thenumber of impacts per sample is reduced as a result of a reduction inthe sample time, then the reduction in sample time exactly compensatesfor the reduced response per sample leaving the latency unchanged.

Radioactivity (number of photons emitted per second) has a very slighteffect on accuracy, improving accuracy only by a factor of 2 as theactivity increases from 0.01 mCi up to 0.5 mCi (FIG. 9B). It has adrastic effect on response time at low activity levels (FIG. 9A) wherethere simply are not enough photons to induce rapid rotation, however atactivity levels above 0.1 mCi there is minimal improvement withincreased activity level. Optimization of this trade-off between latencyand accuracy (see below) is achieved with 0.05 mCi. This specificactivity provides a good compromise between performance and radiationdose, providing a performance suitable for a typical medical applicationwithout imposing a safety risk to the patient or doctor.

In order to optimize the tradeoff between accuracy and latency a motiondetection algorithm was employed to increase the rotation per photonduring motion of tracked source 38. This decreased latency time andincreased accuracy. In the simulation, the percentage of photons hittingreceiving elements 22 classed as positive 21 versus those classed asnegative 23 was used as an indication of motion of tracked source 38. Asthe percentage moved farther away from 50% the rotation per photon wasincreased, reducing latency at the expense of accuracy during motion. Inother words, system 40 begins by moving towards an estimated targetrotation angle 32 in large steps. As estimated target rotation angle 32is approached, the size of the steps is decreased. If target rotationangle 32 is passed, a small compensatory step in the opposite directionis employed. Results are summarized graphically in FIGS. 10 a and 10 b.Briefly, the RMS error of system 40 tracking a moving source 38 is 0.71mm on average. Location of a stationary source 38 by system 40 producesan rms error of 0.62 mm.

In summary, the simulation results indicate that with an activity of0.05 mCi of 192Ir, compound differential sensors of the type illustratedin FIG. 5, and a motion detection algorithm which trades-off latencyagainst accuracy, system 40 can achieve overall accuracy ofapproximately 1 mm RMS.

Simulated sensitivity of sensor module 20 to changes in rotation angle32 is illustrated in FIG. 12 which is a plot of output signal 34 as afunction of rotation relative to target rotation angle 32 for a sensorof the type indicated in FIG. 5. The graph was produced using theformula:

Total Output 34=A/(A+B)

Where A is the sum of all right side sensors 21; and

B is the sum of all left side sensors 23.

The total range of output 34 (Y axis) from sensor 20 was arbitrarilydefined as being in a range from 0 to 1. On the X axis, 0 indicates theangle of rotation 32 which indicates the direction of source 38. Thetotal rotational range of sensor 20 was +32 milliradians from thistarget rotation angle 32. Deviation of more than 32 milliradians awayfrom target rotation angle 32 produced an output 34 of either 0 or 1,indicating the direction of rotation for a return to target rotationangle 32, but not the amount of rotation to reach target rotation angle32. When output 34 is 0 or 1, the only conclusion that can be drawnabout deviation from target rotation angle 32 is that it is greater than32 milliradians in the indicated direction.

The graph of FIG. 12 depicts output 34 for target rotation angle 32 asthe middle of the dynamic range (0.5). If output 34 is 0.4, acorrectional rotation of 10 milliradians in the plus direction isindicated to achieve target rotation angle 32. An output 34 of 0.6indicates a correctional rotation with the same magnitude (10milliradians), but in the minus direction. Another way of depicting thesame information would be to indicate a total dynamic range of +0.5 to−0.5 on the Y axis. This middle of the range could be zero, with onedirection being positive and the other negative, or it can be anyarbitrary number, with one direction being higher and the other lower.

As illustrated in FIG. 12, at target angle 32 simulated sensitivity ofsensor 20 to rotation is approximately 1% of the dynamic range permilliradian of rotation.

This 1% sensitivity per milliradian is sufficient to provide the desiredaccuracy (1 mm rms), using a 5 cm×10 cm sensor module 20 with shields 36having a height 35 of 5 cm interspersed between radiation detectors 22and located 25 cm from source 38 with an activity of 0.05 mCi. Adjustingaccuracy parameters, increasing the size of detectors 22, reducing thedistance between sensor 20 and source 38 and increasing the activity ofsource 38 could each serve to reduce the level of directionalsensitivity desired of sensor 20.

Simulation results (not shown) using a sensor 20 of the type shown inFIG. 6A were similar to those described hereinabove.

FIGS. 22-23 show results of an actual experiment, using a collimator asdescribed with respect to FIGS. 17B and 17C, and a 1 mm spherical sourcehaving 70 μCi of Co57.

Referring to FIG. 22, the smooth line is the actual path, with thejagged line indicating an overlay of the path reconstructed from asignal detected as described herein.

FIG. 23 shows only the error between the actual motion and thereconstructed motion, which error is always smaller than 2 mm and mostof the time smaller than 1 mm.

FIG. 13 is a flowchart 1300 of a method of tracking a radioactive object(38, FIG. 3) based on an angle offset indication, in accordance with anexemplary embodiment of the invention. It is noted that while thedescription focuses on a single (two element) sensor, in someembodiments, multiple sensors are controlled simultaneously for example,three orthogonal sensors.

At 1302, the sensor (20, FIG. 3) is aimed at the object. Optionally, theinitial aim is manual. Alternatively, the initial aim is by scanning thespace using the sensor, for example by rotating or translating thesensor.

At 1304, tracking parameters are optionally set. For example, thetracking parameters may include the expected maximum velocity of theobject or its radiation level and/or a sensitivity plot of the sensor.

At 1306, the sensor generates a signal indicating an angular offsetbetween the aiming line of the sensor and the object.

At 1308, the location of the object in the future is optionallypredicted. Optionally, the prediction relates to the estimated time itwill take the sensor to be realigned. Optionally, the sensor controlmechanism is designed for a certain expected angular motion rate. Thiscan allow a slower control mechanism to be used. Various estimationmethods are known in the art and may be used.

At 1310, the sensor aim is adjusted, for example according to the offsetand/or the predicted position. In the former, the sensor will generallylag behind the source. In the latter, the sensor may lag or be advanced,depending on the correctness of prediction.

In an exemplary embodiment of the invention, the range of angularpositions and the speed of angular change of the sensor are selected sothat the sensor can track a moving object (e.g., maintain inside theangular range), where the maximum speed is known. Possibly, the sensoris never aimed exactly at the object.

FIG. 14 is a schematic figure showing a spatial relationship between atracked object 1418 and an aiming point 1414 of a sensor 1400, inaccordance with an exemplary embodiment of the invention.

Sensor 1400 includes a radiation detector 1402, for example a solidstate detector, which is divided into at least two sections 1410 and1412, optionally separated by a space, optionally an extension of thecollimator slat. A collimator 1404, which will be described in greaterdetail below, is optionally provided to shape the sensitivity of thesensor. (This shaping is not illustrated in FIG. 14). Detector 1402 isoptionally gimbaled using a hinge 1408 relative to a base 1406.

In an exemplary embodiment of the invention, collimator 1404 is designedso that the sensor is most effective over a limited working volume, forexample, that indicated by a dotted area 1416, which includes a range ofdepths and a range of angles. For simplicity, a 2D collimator and spaceis shown, the collimator may define a 3D region, for example, if it is acone-type collimator.

In an exemplary embodiment of the invention, the response of the sensorwithin this working volume varies in accordance with the angular offsetbetween object 1418 and aiming point 1414 (e.g., relative to sensor1400), in a manner which allows the angular offset to be reproduced fromthe response. It should be noted that the shape of the actual workingvolume will depend on the design of the collimator and may be other thanshown, for example, an inverse trapezoid. However, for design purposes,a working volume is defined by a user, for example, a cylinder or acube. It is within this defined working volume that the angular responseis desired. In some cases, the working volume for a sensor is defined byfirst defining a working volume for a set of sensors (e.g., 3) and thenworking back to the individual sensors.

While many collimators can have an angle-varying response, this responsewill often be non-useful in any meaningful area of interest. Somecollimator designs could have an angle-varying response, but thatresponse may be spread over too large (or too small) a range of anglesand/or too small a range of depths. For example, a highly focusedcollimator may provide a meaningful angle-dependent signal for only asingle depth and a narrow range of angles around it. Referring to themethod of FIG. 13, it is desirable in some embodiments of the invention,that the accuracy of the angle determination (e.g., change of signal asa function of change in angle) be relatively uniform (e.g., bounded by afactor of less than 10, less than 5 or less than 2). In an exemplaryembodiment of the invention, the design of the collimator and/ordetector are selected so that the response will be relatively linearwith the angle over a working volume of interest (e.g., dotted area1416).

One type of collimator design which has this behavior is illustrated inFIGS. 15-16, with FIGS. 15A-15C illustrating the angular response andFIG. 16 illustrating features of an exemplary design, in which each oftwo sections 1610 and 1612 has a different aiming (focusing) point(1622, 1620) and the combination of the signals from the two sections isquasi linear over a significant range of angles. As will be described ingreater detail below, a collimator design for achieving a desired effectis optionally generated and/or selected by an optimization process.While the optimization process is described in greater detail withrespect to FIG. 18, some details of exemplary optimization processes aredescribed with reference to FIGS. 15 and 16, to which optimization maybe applied. It is generally desirable that the function be monotonic, toprevent confusion; however, this is not essential, for example, forstepwise non-monotonic functions.

The performance of a single collimated sensor is generally defined bythe shape and amplitude of the plot of the detected signal versus angleof the source. The shape of the plot determines the theoreticalsensitivity of the collimated sensor, and the amplitude determines howaccurately the measurement will correspond to the theoreticalsensitivity, allowing an angle determination to be made from themeasurement.

However, when two such collimated sensors are combined into adifferential sensor (e.g., FIG. 16), there are a number of additionalfactors that are optionally taken into account in determining theperformance:

-   -   1) When combined into a differential sensor, each individual        sensor is not at its peak at the zero angle. The collimators are        optionally (FIG. 16) designed such that their peaks are        off-center and they have a steep slope at the zero angle. By        designing the collimators such that their plots cross at the        zero angle in this steep region, the difference between them        changes sign at the zero angle and has a very steep slope. The        location along their individual signal plots at which the two        collimator signal plots cross will determine the number of        counts per second at the zero angle, which will determine the        accuracy with which the sensor can estimate its angle based on        the recorded differential signal. (e.g., FIG. 15).    -   2) If the sensor is to be used for tracking a moving source        (e.g., FIG. 13), then the differential signal is optionally        designed to provide accurate information about the angle of the        source throughout a range of angles near the zero angle so that        this information can be used to adjust the sensor angle        appropriately after each measurement to most accurately track        the source movement.    -   3) If the sensor is to be used in a mode in which it actually        tracks the movement of a source (e.g., FIG. 13), rather than        performing a sweep through a range of angles, then it is        optionally useful for the sensor to provide an indication of        which direction it must rotate to find the source even when the        source is at a very large angle from the zero angle of the        sensor. This feature may be implemented by requiring that some        small part of each half of the sensor be outside the collimator        so that it receives a signal even at very large angles (e.g.,        FIGS. 5, 6A and 6B).

FIG. 15A shows a composite signal 1506 generated by combining a signal1502 from section 1610 (FIG. 16) and a signal 1504 from a section 1612(FIG. 16). FIG. 15B is an enlarged view of the section of compositesignal 1506 at a significant angular offset, showing error lines(described below) 1510 and an average value 1508. The error shown is adynamic error in estimating the angle at positions other than zerooffset, i.e., the error when the sensor is not pointed at a single mostaccurate focal point (as would be the case for a focused fancollimator). FIG. 15C is a similar enlargement for a zero offset angle,showing a static error, i.e., when the sensor is aimed at the sourcewith no offset. FIGS. 15A-15C show the results of a simulation, wherethe ordinate is in counts and the abscissa is translational offset(equivalent to angle at a distance). Signal 1506 varies over the range−1 . . . +1.

Signal 1506 is generated using a differential sensor output (a-b)/(a+b),where a and b indicate the counts provided by signals 1502 and 1504. Thesignal is normalized to the count. The accuracy of the source positionestimate based on this sensor output generally depends on the number ofcounts measured by each sensor during the measurement period. Thestandard deviation of (a-b)/(a+b) is calculated as:

${std} = {\frac{1}{\sqrt{a + b}} \times \sqrt{1 - ( \frac{a - b}{a + b} )^{2}}}$

It should be noted that this equation is only considered to be accuratewhen a+b is greater than 30 (e.g., that a Poisson distributionapproaches the shape of a Gaussian distribution).

Lines 1510 show one standard deviation.

In an exemplary embodiment of the invention, the angular range of asensor is defined as a maximum rotation, defined, for the left side ofthe graph, as the distance from the zero point until the smaller of twopoints; 1) the point at which a+b falls below 30, or 2) the point atwhich the sensor output +/− one standard deviation reaches +/−1.Generally, the maximum rotation per measurement should be at least aslarge as the maximum source movement per measurement (maximum sourcespeed) in order to enable an estimate of the dynamic error at themaximum source speed. However, this is not essential and may depend onthe application.

The dynamic error is optionally defined as the maximum error within therange of +/−maximum source movement per measurement. The dynamic andstatic errors for any signal output are defined as the maximum positionerror that would result, when the source is off-center or on-centerrespectively, from a signal output of the current signal output +/− onestandard deviation.

In an exemplary embodiment of the invention, a collimator/sensor designis defined by a cost function which indicates a quality of the designfor the parameters of maximum rotation angle, dynamic error and staticerror, for example,

cost function=static error*static error weight+dynamic error*dynamicerror weight+maximum rotation factor

Optionally, a noise level (and/or other error factors) is factored intothe cost function, for example, as part of the error estimation.

In order to ensure that the maximum rotation remains large enough toenable measurement of the dynamic error, the cost function optionallybecomes very large when the maximum rotation falls below the maximumsource movement per measurement. Optionally, the maximum rotation factoris 0 when the maximum rotation is above the range of source movement permeasurement and very quickly becomes extremely large when the maximumrotation is below the maximum source movement per measurement.

Referring again to FIG. 16, parts 1606 and 1608 correspond to parts 1406and 1408 of FIG. 14. A virtual aiming point 1614 is shown, however, nopart of the sensor is adjusted for this point. Instead, part of thesensor is aimed at a left aiming point 1622, which has an offset, forexample, 1-7 mm from point 1614 (in a direction perpendicular to theaiming direction) and part of the sensor is aimed at a right aimingpoint 1620, offset to the right. It should be noted that for a givendepth, angles and translations are interchangeable. As will be describedbelow, the aiming points are optionally part of an optimization processused to define the working volume. While a crossed sensor may beprovided, in which the right side looks left and the left side looksright, this will usually not be done due to reduced efficiency andgeometrical limitations on slats.

In an exemplary embodiment of the invention, this behavior of the sensoris achieved by having the parts of collimator 1604 overlying thesections 1610 and 1612, each as a separate fan collimator or otherfocused collimator. FIG. 17A, below, shows a case where each slat has adifferent angle (aim), with an optional result of a distributed focus.

FIG. 16 shows various collimator parameters which may be adjusted asdescribed below, for example as a result of optimization. “H” is theheight of slat, which may vary between slats, for example as shown. “D”is a distance between slats, which may vary in a collimator, forexample, as described below. “W” is a width of a slat. While a thinnestslat is generally desirable, a minimum width may be required to provideabsorption of radiation and/or structural integrity. “W” may also varywithin a collimator. “F” is the distance to aim point 1622. If multipleaim points are provided, F may be different for different aim points.Optionally, a range of effective “F” values is provided and used todefine a working volume, for example as explained below.

“L” is the offset of the aim point of a sensor portion from the aimpoint of the sensor. Multiple aim points may be provided. In addition, arelatively continuous range of such points (e.g., as shown in FIG. 17A)may be used to define a distributed focus.

FIG. 17A is a schematic diagram showing multiple non-point focal aimingpoints for an alternative differential sensor 1700 in accordance with anexemplary embodiment of the invention.

In sensor 1700, a collimator 1704 comprises a plurality of slats whichare arranged so that each pair of slats has a different aiming point,for example a series of points 1722 for a section 1710 of the sensor anda series of points 1720 for a section 1712 of the sensor. The mainaiming point of the sensor as a whole is shown as 1714. As will bedescribed below, one possible result of such spreading of aiming pointsis control over accuracy in the working volume of the sensor (e.g.,dotted area 1416, FIG. 14). Typically, the width of the series of points1722 and 1720 is smaller than the width of the collimator/detector(e.g., the whole detector or a single one of a pair or more than twodetector sections of a sensor), for example, being less than 50%, lessthan 30%, less than 10%, or intermediate numbers.

In the example shown, while all the slats are angled inwards, for mostof the slats, the angle is less than that of a comparable collimator(with focus at aiming point 1714).

Given, for example, a collimator (and sensor) design with the followingproperties (selected for a depth range of 20-40 cm and a maximum speedof 5 cm/sec):

-   -   a) Slat width: 1.5 mm    -   b) Slat height: 40 mm    -   c) Number of slats: 20    -   d) Spacing between slats: decreasing linearly from 1.5 mm at the        center to 1 mm at the edges    -   e) Sensor depth: 23 mm    -   f) Focal distance: 400 mm

With these collimator geometry parameters, in order to achieve a speedrange up to 5 cm/sec at a distance of 20 cm, the slats can be set up sothat each sensor section is aimed/focused at a maximum offset of 1 mmfrom the center line (e.g., left for the left section, right for theright section). If all slats are arranged for this type of focusing theaccuracy at 20 cm is 0.32 mm and the accuracy at 40 cm is 1.6 mm.

In order to achieve a better than 1 mm accuracy at all distances withinthe range, accuracy at 40 cm should be improved. Optionally this isprovided by readjusting the slats to achieve a defocusing effect, forexample, leaving the center slats arranged for a 1 mm offset, andlinearly increasing the focus offset distance up to 7 mm for the edgeslats. This maintains the maximum speed of 5 cm/sec at 20 cm whileimproving the accuracy at 40 cm to 0.88 mm. The accuracy at 20 cm isslightly reduced to 0.36 mm. Non-linear changes in collimator parametersmay be provided as well. Better accuracy can sometimes be achieved byalso varying the slat heights. Series 1720 and/or 1722 optionally have asignificant range of separation values, for example, 1 mm, 5 mm, 10 mm,20 mm or smaller, intermediate or large values.

It should be noted that the graph of FIG. 15A is correct for aparticular distance. At different distances, the accuracies change.Optionally, the spreading of focuses is used to control theseaccuracies. This generally lowers at least one of the static accuracyand dynamic accuracy. It is noted that a particular feature of someembodiments of the invention is that the behavior of the sensor isrelatively uniform over a range of working distances, for example, therange 20-40 cm as in the example above.

Optionally, the distance of a source from the sensor is determined basedon the sensitivity to angular motion (e.g., of the sensor, by sweeping).Often however, the opposite problem arises, that the mapping of signalto angle depends on the distance. Optionally, a plurality of tablesmatching signals to angles are provided for a plurality of distances(e.g., 10 or more distances). Optionally, a previous position of thetracked device and/or a position estimate provided by other sensors(e.g., 3 sensors may be used) provide at least an estimate of thedistance and thus the table to use for angle estimation. Methods otherthan tables may be used, for example, analytical functions or neuralnetworks.

Multiple aiming points may be provided for other reasons as well. In oneexample, a quadrant detector sensor is provided, in which each of aquadrant of the sensor is aimed/focused at a different x-y offset from acentral aiming point, for example, one quadrant would be (+1, +1),another (+1, −1), and so on. This type of sensor may be used to detect a2D angular offset of a source using a single detector.

In an exemplary embodiment of the invention, the angles of the slats areadjusted so as to allow adjusting the accuracy vs. the speed of theobject. Optionally, individual ones of the signals generated by detectorparts in the detector of FIG. 17A are used to determine what theeffective focus of the detector will be and/or to shape signals 1502,1504 and/or 1506 of FIG. 15A.

FIG. 17B is a schematic illustration of a sensor design in accordancewith an exemplary embodiment of the invention.

As shown, sensor 1750 includes a frame 1752 on which are mounted twoslotted plates 1754. A plurality of lead slats 1756 are arranged by theslots to create collimation for two sensor parts 1758 and 1760. Thedetectors associated with the sensor parts are optionally providedattached to the underside of frame 1752, for example an aluminum frame.

In an exemplary embodiment of the invention, the detectors are 2.5cm×2.5 cm×10 cm in dimension and the collimator is 5 cm×10 cm, withmaximum slat heights of 3.7 cm.

In an exemplary embodiment of the invention, an axis 1762 is providedfor rotating frame 1752. Optionally, the axis is at a center of thedetectors. Optionally, the detectors are curved, but this may not bemeaningful at small angle changes and may adversely affect theuniformity of sensitivity of the detectors. Optionally, a motor 1764 isprovided for rotating axis 1762.

Alternatively or additionally to rotation at the center of the detectorand/or curving of the detector, compensation is made (e.g., in software)for changes in counts due to parts of the detector approaching or movingaway from the source as the detector is rotated and/or due to theeffective thickness of the detector. Optionally, these corrections areincluded in the above described look-up tables.

In an alternative design, the slats are connected by rotating pins (orother hinges) to the plates (not slotted) and the exact relativepositions of the slats is determined by inserting a plastic (or otherradio-transparent material) insert into the collimator, which plasticinsert is a slotted plate machined (or cast) to have desirable relativeslat angles. Optionally, not shown, one or more screws may be providedto adjust the slant angle relative to the slots of the plate or plasticinsert.

FIG. 17C is a side view of sensor 1750, showing changes in slat heights.

Optionally, the center space between the detectors is used to house alight source, for example a collimated laser beam, used to indicate aposition on a target.

Optionally, frame 1752 and/or plates 1754 are rounded, to conform to anoptional cylindrical housing (not shown).

FIG. 18 is a flowchart of a method of collimator optimization and/orselection, in accordance with an exemplary embodiment of the invention.As noted above, collimator/sensor design is, for example, an applicationspecific compromise and may depend on the relative importance of lowerlatency during movement versus increasing accuracy when stationary(FIGS. 15B and 15C).

In the general method shown, at 1802 various parameters are set, forexample, defining what the expected environment is. At 1804, variousconstraints (e.g., on parameters that vary during optimization) and/ordesired quality thresholds or other indicators are set. At 1806, astarting point (e.g., collimator design) is selected.

At 1808, the current collimator design is evaluated. If it is goodenough (1810), this collimator may be chosen (1812). Sufficiency ofdesign may be determined in various manners, for example, based oncertain design thresholds being met or based on lack of progress in theoptimization process.

If the collimator was found lacking, the design is varied (1814) andthis design is evaluated (1808).

Many optimization methods are known in the art and may be applied, forexample, non-linear programming methods, hill climbing and/or exhaustivesearch.

Optionally, the performance of the collimator is evaluated bysimulation. Alternatively or additionally, an analytical calculation isused, for example, based on optical assumptions regarding the radiationand collimator.

There are a number of collimator parameters which can be optimizedthrough the selection of slat geometry. For simplicity of presentation,parameters other than the geometry of the collimator are assumed to befixed. However, this need not be the case and such other parameters mayplay a part in optimization. These fixed parameters include, but neednot be limited to sensor material and geometry, collimator slatmaterial, and/or source energy and activity. In general, theoptimization discussion will focus on slat thickness, placement, andheight, which can all be different for each slat in the collimator. Itis noted, however, that other geometrical properties can be modified,for example, slat shape (e.g., a trapezoid rather than a rectanglecross-section). For simplicity of simulation, variations in slatgeometry were assumed to be limited to linear changes however, this isnot essential and non-linear changes can be provided as well.

The various ranges of values within which a property is allowed to vary,are provided as an input to the optimization but may be changed, forexample, the limits placed on the maximum height, minimum thickness,maximum geometrical resolution, will affect the optimization.

In an exemplary embodiment of the invention, the collimator design isoptimized through the use of a simulator which simulates thedifferential sensor output versus source angle given the properties andgeometries of the collimator and sensor. In an exemplary embodiment ofthe invention, the simulation first constructs a model of the sensorsand collimators, and then calculates the signal output as follows:

-   1) for each of a series of source positions (angles relative to the    sensor), the following procedure is followed:    -   a) A large number of rays are defined, all starting at the        source, and penetrating the sensor with uniformly distributed        coverage.    -   b) For each ray, the total length of collimator slat penetration        is calculated, and the total length of sensor penetration is        calculated.    -   c) From the collimator slat and sensor penetration lengths, a        value is calculated for the percentage of photons along that ray        that would be recorded by the sensor.    -   d) The number of photons per second represented by each ray is        calculated based on the total number of photons given off by the        source, the distance from the source, and the spacing of the        rays.    -   e) The percentage of photons recorded for each ray is averaged        for all of the rays hitting a given sensor and then multiplied        by the number of photons per second represented by each ray to        obtain the simulated number of photons per second recorded by        each sensor.-   2) The number of photons recorded per second by each sensor is    plotted for each of the source positions.-   3) The numbers of photons recorded per second by each sensor are    combined to obtain the differential sensor outputs for each source    position. The differential output used for tracking is generally    (a−b)/(a+b), where a and b are the signal outputs of the two    sensors.

For the purpose of optimizing the collimator and sensor design, thevalue of a+b is optionally used as well, as it indicates the totalnumber of photons per second recorded which can indicate the statisticalaccuracy with which the measured signal can be expected to match thetheoretical signal.

The optimization of the collimator design is optionally achieved throughan iterative minimization technique (1808-1814) in which aperformance-related cost function is minimized by iteratively varyingthe geometric properties of the collimator, running the simulation, andassessing the performance.

It should be noted that for cases where a stationary source is to belocalized, the static error may achieve a high importance (FIG. 15C).Optimization can be achieved for these cases by making the slatsextremely thin and extremely close together and by making extremelysmall sensor rotations. In this way the jitter about the zero angle canbe reduced to a minimum at the expense of very long latency when thesource moves.

In cases where the source is moving the accuracy with which the sourcecan be localized is important, however optimization of the system'sdynamic response is generally desirable—minimizing the tracking errorwhile the source is in motion. In the case of a moving source, due tothe system's inherent response time, the source will not be at the zeroangle at all times. As noted above, optionally the instantaneouslocation of the source at the time of measurement is calculated based onan estimate of the angle from the sensor to the source at the time ofmeasurement.

In an exemplary embodiment of the invention, the system parameters(1802) used to define the requirements for the collimator in the case ofa moving source include one or more of:

-   -   1) range of distances—this defines the minimum and maximum        distance between the sensor and source for which the        optimization should be performed.    -   2) maximum source speed—this defines the maximum speed of the        source for which the dynamic accuracy should be considered (the        maximum speed which the system needs to track accurately for a        given tracking cycle, which may depend on the sensitivity of        radiation detection). This value is given as the maximum        movement of the source per measurement. In a perfect system,        where a point source (rather than multiple sources) are viewed        and knowing the refresh rate, this defines the angular range as        well.    -   3) dynamic error—this defines the accuracy with which the sensor        must estimate the current position of the source when at its        maximum speed.    -   4) static error—this defines the accuracy with which the sensor        must estimate the current position of the source when standing        still.    -   5) optionally, system response speed which models an imperfect        motion mechanism for the sensor, for example, including delay.

The range of distances and maximum source speed are absolute parametersthat define the ranges of operation within which the optimization isperformed. The dynamic and static error parameters are weights whichdefine the relative importance of these parameters for a particularapplication. The algorithm uses these weights to trade off among theseparameters in order to achieve an optimum collimator for theapplication.

For the purpose of describing the optimization methodology, a typicalset of parameter values is chosen, but it should be clear that thevalues of these parameters are application dependant and they must bedefined appropriately for every application.

For the application of tracking the position of the tip of a medicaldevice within a patient's body the following system parameter values areused:

-   -   1) range of distances: 150-300 mm    -   2) maximum source speed: 5 mm/measurement (5 cm/sec, 10 Hz        measurement rate)    -   3) dynamic error weight: 1    -   4) static error weight: 2

The geometrical parameters to be optimized can be selected andconstrained (1804) based on the needed level of optimization and/or thecollimator construction methods available. For the purpose ofdemonstrating the optimization methodology, a relatively simple set ofparameters and constraints is selected:

Parameters which are allowed to vary:

-   -   1) slat thickness    -   2) slat height    -   3) slat spacing    -   4) collimator focal distance (the Y position of the focal        point—its distance from the sensor)    -   5) collimator focal offset (the X position of the focal        point—its distance from the zero plane/line of the sensor        aiming)

Constraints:

-   -   1) minimum slat thickness: 1 mm    -   2) maximum slat height: 40 mm    -   3) geometrical resolution (accuracy to which thickness and        spacing can be defined): 0.1 mm    -   4) thickness, height, and spacing of consecutive slats change        linearly.    -   5) All slats on each half of the collimator are angled to focus        on a single focal point located at the focal distance and focal        offset. The slats in the two halves of the differential        collimator will focus on different points, one with a positive        offset from the zero plane, and one with a negative offset from        the zero plane. As noted with respect to FIG. 17A, the angle may        be varied between slats, for example, in a linear manner.

The actual parameters optimized by the optimization algorithm willgenerally depend upon the constraints. In this case, since the slatgeometries are constrained to change linearly from slat to slat, each ofthe geometrical properties can be represented by two values, the valuefor the first slat and a linear factor to be applied to each consecutiveslat. Since all slats on each half of the collimator are constrained tofocus on a single focal point, the angles of all slats can be defined bytwo values; the focal distance and the focal offset. In this case thereare 8 parameters to optimize:

-   -   1) first slat thickness    -   2) slat thickness linear factor    -   3) first slat height    -   4) slat height linear factor    -   5) first slat spacing    -   6) slat spacing linear factor    -   7) collimator focal distance    -   8) collimator focal offset

The number of slats will be the number of slats that fit on the givensensor for each set of geometric parameter values.

In the above description, the movement of the source is assumed to beconstant. Optionally, the simulation takes into account actual usageparameters. For example, the simulation may use a set or range ofcatheter motions and/or speeds, to compare collimator designs.Optionally, the simulation generates a quality for a collimator based onits overall behavior in a scenario or set of scenarios. For example, afirst collimator may have a smaller average error but a greater maximumerror than another design. However, if this maximum error is in a partof the path which is indicated as being less important and/or forlimited conditions, the first design may be preferred.

Optionally, the simulation (e.g., source speed and/or paths) takes intoaccount patient behavior (e.g., breathing and other natural body motion)and/or movement of the patient, for example, fidgeting.

Optionally, the optimization takes into account an angular range neededto be viewed at a same time, for example, to allow a detector tosimultaneously receive signals from multiple radioactive sources on asingle object (e.g., two sources with different energies on a same toolsuch as a catheter). Detection of the relative position of the sourcesis optionally used to determine an orientation of the tool.

Alternatively or additionally, the simulation can include a simulationof a tracking behavior by a modeled system, for example, with giventracking speed and/or tracking frequency. Optionally, such parameters ofthe system (e.g., tracking speed and/or frequency) are parameters whichmay be optimized by the simulation.

Optionally, detector design is part of the optimization, for example,including a parameter defining an accuracy of positional determinationof a count within an area between two slats. This may be provided, forexample, using a CCD array imager instead of a scintillation detector,to view the detector material (e.g., a doped halide crystal).

Optionally, for a given collimator/detector/system, a calibrationenvelope value (or set) is generated indicating maximum allowed speeds,angles, distance, etc. when, in actual use the envelope is exceeded orabout to be exceeded, a user is optionally alerted. Optionally, a usercan indicate a desired accuracy and the system will generate a warningwhen this desired accuracy cannot be achieved. Optionally, a calibrationprocess is carried out where simulation results are normalized, forexample, according to actual noise levels and/or source behavior.

In some cases, the application specific optimization takes into accounttime constraints. For example, even though a tracking algorithm isdescribed, at some times, a scanning mechanism may be allowed. Forexample, the application may allow a user to pause a few times for asecond or a fraction thereof (e.g., in response to a beep by the system)and allow the system to re-track.

It should be noted that while the above method can be used to generatean “optimal” or near optimal design for a collimator, optionally, themethod is used to select from a set of available collimators.

Optionally, a set of collimators is provided, each one suitable for adifferent application and a software application or table is providedwhich matches up a best collimator with the application parameters. Inan exemplary embodiment of the invention, a set of collimators includesbetween 4 and 10 collimators. Optionally, for a given application, 2-3collimators are provided, for example, each one optimized for adifferent range of distances. In addition, applications may be looselydivided up into applications with very slow motion, intermediate speed(e.g., breathing motion) and high speed (e.g., catheter motion).Separate collimator sets may be provided for each such application. Itshould be noted that for high-speed detection of position, higherradioactivity may be desirable, than for low speeds.

Various collimators can be produced according to the methods describedherein. In one example, the collimator is designed for a range of anglessmaller than 150, 100, 80, or fewer milliradians. Alternatively oradditionally, the collimator is designed for a range of angles of atleast 5, at least 20, at least 30, at least 40, at least 50, or more orintermediate milliradians.

Alternatively or additionally, the collimator is designed for a range ofdepths, for example, at least 10 cm, at least 20 cm, at least 30 cm, atleast 40 cm, at least 50 cm or intermediate values. Optionally, thecollimator is designed for a range smaller than 100 cm, smaller than 80cm, smaller than 50 cm or smaller than 30 cm. The width of the workingvolume is optionally similar to the depth range, but it may be greater,for example, being within 70%, 90% or smaller, intermediate or greaterpercentages of the detector length.

In an exemplary embodiment of the invention, the detector is smallerthan 20 cm×20 cm or smaller than 10 cm×30 cm. Optionally, the workingvolume is within a ratio of 1:4 of the detector dimensions multiplied bythe detector length. In an exemplary embodiment of the invention, thedepth of the working volume is within a range of 1:10 of the width ofthe detector.

In an exemplary embodiment of the invention, the slope of the signal(1506) is stable to within a ratio of 1:1.5, 1:2, 1:3 or intermediateratios over the working volume.

While multiple detector portions may be provided, optionally, the sensoris non-imaging. Optionally, the lack of imaging is inherent in thesignal that is generated. Optionally, lack of imaging is provided byhaving fewer than 50, fewer than 20, fewer than 10 or intermediatenumbers of separately read detector elements.

System 40 and/or sensors 20 rely upon execution of various commands andanalysis and translation of various data inputs. Any of these commands,analyses or translations may be accomplished by software, hardware orfirmware according to various alternative embodiments. In an exemplaryembodiment of the invention, machine readable media contain instructionsfor transforming output signal 34 from one or more sensor modules 20into position co-ordinates of source 38, optionally according to method400. In an exemplary embodiment of the invention, CPU 42 executesinstructions for transforming output signal 34 from one or more sensormodules 20 into position co-ordinates of source 38, optionally accordingto method 400.

According to an exemplary embodiment of the invention a trackablemedical device is manufactured by incorporating into or fixedlyattaching a detectable amount of a radioactive isotope to the medicaldevice. The radioactive isotope may or may not have a medical functionaccording to various embodiments. Optionally, the radioactivity of theisotope has no medical function. Optionally, the radioactive isotope maybe selected so that it can be used in the body without a protectivecoating without adverse reaction with tissue. In an exemplary embodimentof the invention, the detectable amount of isotope is in the range of0.5 mCi to 0.001 mCi. Use of isotope source 38 with an activity in thelower portion of this range may depend on lower speeds of the device,sensitivity of detector(s) 22, distance from sensor 20. Optionally, atleast 1, optionally at least 5, optionally at least 10, optionally atleast 100 detectable counts per second are produced by the incorporatedradioactive isotope.

In the description and claims of the present application, each of theverbs “comprise”, “include” and “have” as well as any conjugatesthereof, are used to indicate that the object or objects of the verb arenot necessarily a complete listing of members, components, elements orparts of the subject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to necessarily limit the scope of the invention. The describedembodiments comprise different features, not all of which are requiredin all embodiments of the invention. Some embodiments of the inventionutilize only some of the features or possible combinations of thefeatures. Variations of embodiments of the present invention that aredescribed and embodiments of the present invention comprising differentcombinations of features noted in the described embodiments can becombined in all possible combinations including, but not limited to useof features described in the context of one embodiment in the context ofany other embodiment. The scope of the invention is limited only by thefollowing claims.

1-34. (canceled)
 35. A system for determining a position for each of oneor more concentrated radioactive sources, the system comprising: a) oneor more multi-focal sensors, each comprising: i) a detector comprisingat least two distinguishable sections; and ii) a collimator arranged todifferently collimate radiation on each of said sections; and b) acomputerized processing unit which is configured to receive data fromthe one or more sensors and to determine the position of each of the oneor more sources, not using imaging circuitry.
 36. A system according toclaim 35, wherein at least one sensor comprises circuitry coupled to itsdetector which, when there is only one radioactive source, generates anoutput signal which varies as a function of relative orientation of thesensor to the radioactive source, and wherein, for said sensor, saiddetector and said collimator are arranged to have a working volume of atleast 10 cm in depth and having an angular width, such that said signaldefines an accuracy of better than 3 mm within one standard deviation,over said working volume.
 37. A system according to claim 36, whereinsaid accuracy is better than 2 mm.
 38. A system according to claim 37,wherein said accuracy is better than 1 mm.
 39. A system according toclaim 36, wherein said signal is near linear over said working volume.40. A system according to claim 36, wherein a ratio between an accuracywhen said sensor is aimed at said source and when said sensor is at anangle within said working volume, is between 1:4 and 4:1.
 41. A systemaccording to claim 36, wherein said working volume has an angular rangeof at least 10 milliradians.
 42. A system according to claim 36, whereinsaid working volume has an angular range of at least 20 milliradians.43. A system according to claim 36, wherein said circuitry generatessaid signal based on a combining of contributions of at least twoseparate sections of said detector.
 44. A system according to claim 43wherein said two sections each have different angular direction ofmaximum detection.
 45. A system according to claim 36, wherein saidcircuitry generates said signal for a source distance of at least 10 cm.46. A system according to claim 36, wherein said circuitry generatessaid signal for a source distance of at least 20 cm.
 47. A systemaccording to claim 36, wherein said working volume has a range of depthshaving a ratio of at least 1:2.
 48. A system according to any of claim35, comprising, for at least one sensor, a motor configured to rotatesaid sensor and change its aim thereby.
 49. A system according to claim48, wherein the computerized processing unit is adapted to control themotor to rotate the sensor in a direction and/or to a degree thatdepends on data from the one or more sensors.
 50. A system according toclaim 35, wherein for at least one sensor, the detector comprises twosections, each one with a different focal area.
 51. A system accordingto claim 35, wherein for at least one sensor, said collimator providesmultiple focal points for each of said sections.
 52. A system accordingto claim 35, wherein for at least one sensor, said collimator allowswide angle radiation at a spatial angle of at least 10 degrees for atleast two sections.
 53. A system according to claim 35, wherein for atleast one sensor, a focal point of a first section is distanced from afocal point of a second section in a direction parallel to saiddetector, a distance of at least 1 mm.
 54. A system according to claim53, wherein the sensor comprises additional sections with additionalfocal points distanced along said parallel direction.
 55. A systemaccording to claim 53, wherein said sensor has a relatively linearangular response over an angle range of at least 10 milliradians.
 56. Asystem according to claim 53, wherein said sensor has a relativelylinear angular response over a depth range of at least 10 cm.
 57. Asystem according to claim 35, comprising fewer than 50 separately readdetector sections.
 58. A system according to claim 57, comprising fewerthan 20 separately read detector sections.
 59. A system according toclaim 58, comprising fewer than 10 separately read detector sections.60. A system according to claim 35, wherein the computerized processingunit is adapted to use differential data from the two sections of atleast one sensor to determine the positions of the centers of mass ofthe one or more sources.
 61. A system according to claim 35, wherein theone or more sensors are not connected to imaging circuitry capable ofusing the data to reconstruct an image of non-source features.
 62. Asystem according to claim 35, where the computerized processing unit isconfigured to calculate the position of the center of mass of each ofthe one or more sources.
 63. A method of collimator design for adetector for tracking a radioactive source, the method comprising: a)defining a range of movement rate of the source; b) defining aperformance-related cost function depending on a dynamic error and astatic error; c) choosing a collimator design; d) calculating the costfunction for the chosen collimator design, range, and movement rate,using a simulation, an analytic calculation, or both; and e) repeating(c) and (d) at least once to find a collimator design with lower costfunction.
 64. A method according to claim 63, wherein repeating at leastonce comprises repeating until an optimized design is found.
 65. Amethod according to claim 63, wherein the detector comprises first andsecond sections, and choosing a collimator design comprises choosing adesign with collimators arranged to differently collimate radiation forthe first and second sections.
 66. A method of tracking a radioactiveobject, comprising: (a) aiming at least one sensor at said object; (b)detecting an angular offset of said object from said sensor, based on aradiation detection by said sensor; (c) re-aiming said sensor at saidobject according to said angular offset by automatic circuitry; and (d)repeating (b)-(c) at least 10 times within a minute.
 67. A methodaccording to claim 66, wherein the sensor comprises: i) a detectorcomprising at least two distinguishable sections; and ii) a collimatorarranged to differently collimate radiation on each of said sections.68. A method according to claim 66, wherein said re-aiming does not aimsaid sensor exactly at said object at least 50% of the time.
 69. Amethod according to claim 66, wherein said re-aiming comprises anestimate of a current position of the object.
 70. A method according toclaim 66, wherein said re-aiming comprises an estimate of a futureposition of the object.
 71. An angle-responsive sensor, comprising: aradiation detector adapted to detect ionizing radiation; at least oneradiation absorbing element arranged to block radiation from reachingsaid detector in a manner dependent on a relative orientation of aradiation source, said detector and said element, said detector and saidelement defining an aim for said sensor; and circuitry coupled to saiddetector and which generates an output signal which varies as a functionof said relative orientation, wherein said detector and said element arearranged to have a working volume of at least 10 cm in depth and havingan angular width, such that said signal defines an accuracy of betterthan 3 mm within one standard deviation, over said working volume.
 72. Amethod of collimator design, comprising: defining an object range andmovement rate; and determining a collimator design responsive to saiddefining which has a linear-like angular response within said range andsuitable for tracking said movement rate.
 73. A method according toclaim 72, comprising generating a collimator according to saiddetermining.
 74. A method according to claim 72, comprising selecting acollimator according to said determining.
 75. A method according toclaim 72, wherein determining comprises determining in response to adesired accuracy of said angular response.
 76. A collimator set,comprising: at least two collimators, each collimator having a betterangular-accuracy under a different set of conditions, each set ofconditions defining a depth and an angular range, said two setsdiffering in at least one of depth and angular range, said angularranges being greater than 10 milliradians.
 77. A collimator setaccording to claim 76, including collimators for at least threedifferent angular ranges.
 78. A collimator set according to claim 76,including collimators for at least three different depths.